Travis: enable apt caching
Travis: enable apt caching

This should speed up the installation process a bit.

yogo1212/RIOT,biboc/RIOT,Lotterleben/RIOT,dhruvvyas90/RIOT,jferreir/RIOT,kYc0o/RIOT,msolters/RIOT,lebrush/RIOT,dkm/RIOT,avmelnikoff/RIOT,lebrush/RIOT,avmelnikoff/RIOT,jferreir/RIOT,sumanpanchal/RIOT,jbeyerstedt/RIOT-OTA-update,sgso/RIOT,kYc0o/RIOT,Lotterleben/RIOT,ximus/RIOT,roberthartung/RIOT,LudwigOrtmann/RIOT,1blankz7/RIOT,ximus/RIOT,basilfx/RIOT,immesys/RiSyn,MonsterCode8000/RIOT,attdona/RIOT,LudwigOrtmann/RIOT,sgso/RIOT,thomaseichinger/RIOT,asanka-code/RIOT,authmillenon/RIOT,abp719/RIOT,rfuentess/RIOT,smlng/RIOT,backenklee/RIOT,rfswarm2/RIOT,altairpearl/RIOT,alex1818/RIOT,plushvoxel/RIOT,mziegert/RIOT,roberthartung/RIOT,Lexandro92/RIOT-CoAP,yogo1212/RIOT,MohmadAyman/RIOT,rajma996/RIOT,miri64/RIOT,centurysys/RIOT,RIOT-OS/RIOT,dhruvvyas90/RIOT,altairpearl/RIOT,brettswann/RIOT,altairpearl/RIOT,ThanhVic/RIOT,marcosalm/RIOT,lazytech-org/RIOT,LudwigKnuepfer/RIOT,RBartz/RIOT,A-Paul/RIOT,ks156/RIOT,jfischer-phytec-iot/RIOT,Hyungsin/RIOT-OS,d00616/RIOT,tdautc19841202/RIOT,bartfaizoltan/RIOT,Ell-i/RIOT,OTAkeys/RIOT,Ell-i/RIOT,locicontrols/RIOT,thiagohd/RIOT,rajma996/RIOT,Hyungsin/RIOT-OS,rfswarm/RIOT,Hyungsin/RIOT-OS,RubikonAlpha/RIOT,gebart/RIOT,thiagohd/RIOT,locicontrols/RIOT,authmillenon/RIOT,nsol-nmsu/RIOT,shady33/RIOT,backenklee/RIOT,ks156/RIOT,MohmadAyman/RIOT,TobiasFredersdorf/RIOT,marcosalm/RIOT,toonst/RIOT,zhuoshuguo/RIOT,rajma996/RIOT,smlng/RIOT,Osblouf/RIOT,jhollister/RIOT,kb2ma/RIOT,syin2/RIOT,ant9000/RIOT,binarylemon/RIOT,d00616/RIOT,josephnoir/RIOT,katezilla/RIOT,kaspar030/RIOT,TobiasFredersdorf/RIOT,rfuentess/RIOT,tdautc19841202/RIOT,shady33/RIOT,wentaoshang/RIOT,mfrey/RIOT,malosek/RIOT,abp719/RIOT,neumodisch/RIOT,katezilla/RIOT,DipSwitch/RIOT,FrancescoErmini/RIOT,biboc/RIOT,LudwigOrtmann/RIOT,Darredevil/RIOT,thomaseichinger/RIOT,abkam07/RIOT,rfswarm2/RIOT,koenning/RIOT,BytesGalore/RIOT,attdona/RIOT,JensErdmann/RIOT,dailab/RIOT,backenklee/RIOT,nsol-nmsu/RIOT,jremmert-phytec-iot/RIOT,dailab/RIOT,ThanhVic/RIOT,robixnai/RIOT,LudwigKnuepfer/RIOT,RBartz/RIOT,ntrtrung/RIOT,katezilla/RIOT,haoyangyu/RIOT,dkm/RIOT,sumanpanchal/RIOT,authmillenon/RIOT,RubikonAlpha/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,MarkXYang/RIOT,d00616/RIOT,jferreir/RIOT,immesys/RiSyn,latsku/RIOT,koenning/RIOT,marcosalm/RIOT,jhollister/RIOT,kerneltask/RIOT,haoyangyu/RIOT,beurdouche/RIOT,robixnai/RIOT,OlegHahm/RIOT,ximus/RIOT,changbiao/RIOT,RubikonAlpha/RIOT,jremmert-phytec-iot/RIOT,benoit-canet/RIOT,adrianghc/RIOT,herrfz/RIOT,tfar/RIOT,MonsterCode8000/RIOT,immesys/RiSyn,Osblouf/RIOT,khhhh/RIOT,ks156/RIOT,EmuxEvans/RIOT,miri64/RIOT,Darredevil/RIOT,gebart/RIOT,smlng/RIOT,yogo1212/RIOT,Lotterleben/RIOT,kaleb-himes/RIOT,LudwigOrtmann/RIOT,josephnoir/RIOT,kYc0o/RIOT,lazytech-org/RIOT,rfswarm/RIOT,rousselk/RIOT,jhollister/RIOT,AnonMall/RIOT,gebart/RIOT,kushalsingh007/RIOT,fnack/RIOT,EmuxEvans/RIOT,ximus/RIOT,luciotorre/RIOT,OTAkeys/RIOT,MonsterCode8000/RIOT,plushvoxel/RIOT,gbarnett/RIOT,ant9000/RIOT,benoit-canet/RIOT,thiagohd/RIOT,kaspar030/RIOT,kerneltask/RIOT,syin2/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,gbarnett/RIOT,jremmert-phytec-iot/RIOT,AnonMall/RIOT,zhuoshuguo/RIOT,attdona/RIOT,jhollister/RIOT,rajma996/RIOT,openkosmosorg/RIOT,stevenj/RIOT,attdona/RIOT,mtausig/RIOT,jasonatran/RIOT,locicontrols/RIOT,EmuxEvans/RIOT,attdona/RIOT,Yonezawa-T2/RIOT,ThanhVic/RIOT,binarylemon/RIOT,ant9000/RIOT,fnack/RIOT,tdautc19841202/RIOT,neumodisch/RIOT,beurdouche/RIOT,mfrey/RIOT,latsku/RIOT,khhhh/RIOT,sgso/RIOT,herrfz/RIOT,roberthartung/RIOT,Yonezawa-T2/RIOT,RBartz/RIOT,ximus/RIOT,Darredevil/RIOT,mfrey/RIOT,RBartz/RIOT,adjih/RIOT,aeneby/RIOT,mziegert/RIOT,A-Paul/RIOT,Josar/RIOT,kaleb-himes/RIOT,syin2/RIOT,dailab/RIOT,lebrush/RIOT,1blankz7/RIOT,MarkXYang/RIOT,aeneby/RIOT,gautric/RIOT,immesys/RiSyn,FrancescoErmini/RIOT,Lotterleben/RIOT,brettswann/RIOT,rousselk/RIOT,TobiasFredersdorf/RIOT,kaspar030/RIOT,jasonatran/RIOT,emmanuelsearch/RIOT,x3ro/RIOT,jferreir/RIOT,PSHIVANI/Riot-Code,asanka-code/RIOT,openkosmosorg/RIOT,cladmi/RIOT,changbiao/RIOT,abkam07/RIOT,aeneby/RIOT,dkm/RIOT,rfuentess/RIOT,robixnai/RIOT,centurysys/RIOT,khhhh/RIOT,FrancescoErmini/RIOT,asanka-code/RIOT,Yonezawa-T2/RIOT,arvindpdmn/RIOT,fnack/RIOT,changbiao/RIOT,basilfx/RIOT,Josar/RIOT,Josar/RIOT,BytesGalore/RIOT,automote/RIOT,changbiao/RIOT,msolters/RIOT,l3nko/RIOT,JensErdmann/RIOT,sgso/RIOT,LudwigKnuepfer/RIOT,Lotterleben/RIOT,arvindpdmn/RIOT,arvindpdmn/RIOT,josephnoir/RIOT,yogo1212/RIOT,x3ro/RIOT,kushalsingh007/RIOT,JensErdmann/RIOT,gbarnett/RIOT,rousselk/RIOT,openkosmosorg/RIOT,phiros/RIOT,automote/RIOT,kbumsik/RIOT,dkm/RIOT,aeneby/RIOT,chris-wood/RIOT,RIOT-OS/RIOT,LudwigKnuepfer/RIOT,DipSwitch/RIOT,emmanuelsearch/RIOT,beurdouche/RIOT,wentaoshang/RIOT,Ell-i/RIOT,mziegert/RIOT,hamilton-mote/RIOT-OS,ant9000/RIOT,tfar/RIOT,DipSwitch/RIOT,gautric/RIOT,Ell-i/RIOT,kb2ma/RIOT,JensErdmann/RIOT,BytesGalore/RIOT,watr-li/RIOT,jasonatran/RIOT,patkan/RIOT,1blankz7/RIOT,gebart/RIOT,kaleb-himes/RIOT,authmillenon/RIOT,adjih/RIOT,koenning/RIOT,MonsterCode8000/RIOT,locicontrols/RIOT,chris-wood/RIOT,kushalsingh007/RIOT,sumanpanchal/RIOT,miri64/RIOT,adrianghc/RIOT,mtausig/RIOT,emmanuelsearch/RIOT,biboc/RIOT,openkosmosorg/RIOT,zhuoshuguo/RIOT,sgso/RIOT,rakendrathapa/RIOT,openkosmosorg/RIOT,alex1818/RIOT,cladmi/RIOT,ThanhVic/RIOT,changbiao/RIOT,rakendrathapa/RIOT,RBartz/RIOT,arvindpdmn/RIOT,watr-li/RIOT,herrfz/RIOT,dkm/RIOT,tdautc19841202/RIOT,cladmi/RIOT,haoyangyu/RIOT,RIOT-OS/RIOT,centurysys/RIOT,backenklee/RIOT,MohmadAyman/RIOT,rfswarm2/RIOT,locicontrols/RIOT,ThanhVic/RIOT,wentaoshang/RIOT,l3nko/RIOT,fnack/RIOT,jasonatran/RIOT,dhruvvyas90/RIOT,Lexandro92/RIOT-CoAP,zhuoshuguo/RIOT,neiljay/RIOT,nsol-nmsu/RIOT,dhruvvyas90/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,arvindpdmn/RIOT,neumodisch/RIOT,PSHIVANI/Riot-Code,jremmert-phytec-iot/RIOT,luciotorre/RIOT,abkam07/RIOT,jhollister/RIOT,jbeyerstedt/RIOT-OTA-update,khhhh/RIOT,rajma996/RIOT,openkosmosorg/RIOT,neiljay/RIOT,sumanpanchal/RIOT,avmelnikoff/RIOT,gbarnett/RIOT,RIOT-OS/RIOT,stevenj/RIOT,syin2/RIOT,brettswann/RIOT,daniel-k/RIOT,patkan/RIOT,kaleb-himes/RIOT,RubikonAlpha/RIOT,1blankz7/RIOT,thomaseichinger/RIOT,neiljay/RIOT,biboc/RIOT,AnonMall/RIOT,gautric/RIOT,tfar/RIOT,herrfz/RIOT-old,authmillenon/RIOT,tdautc19841202/RIOT,OTAkeys/RIOT,herrfz/RIOT-old,stevenj/RIOT,koenning/RIOT,altairpearl/RIOT,kerneltask/RIOT,Osblouf/RIOT,immesys/RiSyn,jbeyerstedt/RIOT-OTA-update,rfswarm2/RIOT,smlng/RIOT,rfswarm/RIOT,chris-wood/RIOT,jremmert-phytec-iot/RIOT,centurysys/RIOT,mtausig/RIOT,ntrtrung/RIOT,neumodisch/RIOT,luciotorre/RIOT,basilfx/RIOT,lazytech-org/RIOT,Hyungsin/RIOT-OS,lebrush/RIOT,asanka-code/RIOT,rousselk/RIOT,LudwigOrtmann/RIOT,malosek/RIOT,Yonezawa-T2/RIOT,kbumsik/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,lebrush/RIOT,DipSwitch/RIOT,adjih/RIOT,dhruvvyas90/RIOT,kerneltask/RIOT,rousselk/RIOT,benoit-canet/RIOT,alignan/RIOT,alignan/RIOT,Darredevil/RIOT,ntrtrung/RIOT,katezilla/RIOT,plushvoxel/RIOT,cladmi/RIOT,shady33/RIOT,kbumsik/RIOT,ks156/RIOT,malosek/RIOT,msolters/RIOT,fnack/RIOT,MonsterCode8000/RIOT,daniel-k/RIOT,alignan/RIOT,stevenj/RIOT,emmanuelsearch/RIOT,stevenj/RIOT,wentaoshang/RIOT,latsku/RIOT,dhruvvyas90/RIOT,jfischer-phytec-iot/RIOT,mtausig/RIOT,watr-li/RIOT,patkan/RIOT,benoit-canet/RIOT,roberthartung/RIOT,altairpearl/RIOT,AnonMall/RIOT,adjih/RIOT,avmelnikoff/RIOT,ks156/RIOT,daniel-k/RIOT,backenklee/RIOT,JensErdmann/RIOT,lazytech-org/RIOT,abp719/RIOT,neumodisch/RIOT,authmillenon/RIOT,PSHIVANI/Riot-Code,robixnai/RIOT,malosek/RIOT,OlegHahm/RIOT,brettswann/RIOT,alignan/RIOT,abkam07/RIOT,chris-wood/RIOT,gbarnett/RIOT,wentaoshang/RIOT,FrancescoErmini/RIOT,plushvoxel/RIOT,herrfz/RIOT-old,x3ro/RIOT,herrfz/RIOT,wentaoshang/RIOT,adrianghc/RIOT,PSHIVANI/Riot-Code,ntrtrung/RIOT,aeneby/RIOT,MarkXYang/RIOT,attdona/RIOT,phiros/RIOT,tfar/RIOT,toonst/RIOT,toonst/RIOT,Osblouf/RIOT,shady33/RIOT,Lexandro92/RIOT-CoAP,benoit-canet/RIOT,AnonMall/RIOT,rfuentess/RIOT,marcosalm/RIOT,rajma996/RIOT,automote/RIOT,automote/RIOT,msolters/RIOT,yogo1212/RIOT,abp719/RIOT,robixnai/RIOT,kb2ma/RIOT,arvindpdmn/RIOT,abkam07/RIOT,jfischer-phytec-iot/RIOT,jhollister/RIOT,locicontrols/RIOT,rousselk/RIOT,FrancescoErmini/RIOT,alex1818/RIOT,RBartz/RIOT,MonsterCode8000/RIOT,msolters/RIOT,brettswann/RIOT,luciotorre/RIOT,rfswarm/RIOT,sumanpanchal/RIOT,bartfaizoltan/RIOT,alignan/RIOT,tdautc19841202/RIOT,OlegHahm/RIOT,Yonezawa-T2/RIOT,MarkXYang/RIOT,gautric/RIOT,Josar/RIOT,stevenj/RIOT,haoyangyu/RIOT,alex1818/RIOT,MarkXYang/RIOT,locicontrols/RIOT,Darredevil/RIOT,MarkXYang/RIOT,tfar/RIOT,basilfx/RIOT,yogo1212/RIOT,mziegert/RIOT,kb2ma/RIOT,kYc0o/RIOT,biboc/RIOT,sumanpanchal/RIOT,basilfx/RIOT,A-Paul/RIOT,d00616/RIOT,hamilton-mote/RIOT-OS,emmanuelsearch/RIOT,1blankz7/RIOT,adjih/RIOT,thomaseichinger/RIOT,JensErdmann/RIOT,Lotterleben/RIOT,bartfaizoltan/RIOT,EmuxEvans/RIOT,OlegHahm/RIOT,plushvoxel/RIOT,x3ro/RIOT,malosek/RIOT,PSHIVANI/Riot-Code,shady33/RIOT,neiljay/RIOT,kaleb-himes/RIOT,automote/RIOT,immesys/RiSyn,l3nko/RIOT,Yonezawa-T2/RIOT,mfrey/RIOT,emmanuelsearch/RIOT,nsol-nmsu/RIOT,phiros/RIOT,kbumsik/RIOT,rakendrathapa/RIOT,adrianghc/RIOT,RubikonAlpha/RIOT,thiagohd/RIOT,latsku/RIOT,RubikonAlpha/RIOT,LudwigKnuepfer/RIOT,nsol-nmsu/RIOT,dailab/RIOT,Lotterleben/RIOT,l3nko/RIOT,phiros/RIOT,lebrush/RIOT,mziegert/RIOT,syin2/RIOT,toonst/RIOT,josephnoir/RIOT,hamilton-mote/RIOT-OS,daniel-k/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,herrfz/RIOT,l3nko/RIOT,EmuxEvans/RIOT,kbumsik/RIOT,beurdouche/RIOT,rakendrathapa/RIOT,jfischer-phytec-iot/RIOT,kushalsingh007/RIOT,mfrey/RIOT,phiros/RIOT,centurysys/RIOT,beurdouche/RIOT,Lexandro92/RIOT-CoAP,AnonMall/RIOT,rfswarm/RIOT,chris-wood/RIOT,binarylemon/RIOT,herrfz/RIOT,jbeyerstedt/RIOT-OTA-update,fnack/RIOT,jasonatran/RIOT,khhhh/RIOT,ntrtrung/RIOT,marcosalm/RIOT,altairpearl/RIOT,lazytech-org/RIOT,latsku/RIOT,automote/RIOT,OTAkeys/RIOT,zhuoshuguo/RIOT,rfswarm2/RIOT,rfuentess/RIOT,BytesGalore/RIOT,MohmadAyman/RIOT,marcosalm/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,zhuoshuguo/RIOT,luciotorre/RIOT,jferreir/RIOT,rakendrathapa/RIOT,haoyangyu/RIOT,ntrtrung/RIOT,mtausig/RIOT,phiros/RIOT,asanka-code/RIOT,Hyungsin/RIOT-OS,rfswarm2/RIOT,patkan/RIOT,bartfaizoltan/RIOT,EmuxEvans/RIOT,hamilton-mote/RIOT-OS,daniel-k/RIOT,FrancescoErmini/RIOT,kerneltask/RIOT,alex1818/RIOT,DipSwitch/RIOT,d00616/RIOT,gbarnett/RIOT,rfswarm/RIOT,A-Paul/RIOT,binarylemon/RIOT,TobiasFredersdorf/RIOT,kb2ma/RIOT,abp719/RIOT,haoyangyu/RIOT,thiagohd/RIOT,Osblouf/RIOT,josephnoir/RIOT,abkam07/RIOT,Lexandro92/RIOT-CoAP,koenning/RIOT,kushalsingh007/RIOT,TobiasFredersdorf/RIOT,Josar/RIOT,d00616/RIOT,jbeyerstedt/RIOT-OTA-update,binarylemon/RIOT,toonst/RIOT,neumodisch/RIOT,daniel-k/RIOT,PSHIVANI/Riot-Code,gautric/RIOT,shady33/RIOT,changbiao/RIOT,benoit-canet/RIOT,kYc0o/RIOT,centurysys/RIOT,x3ro/RIOT,thomaseichinger/RIOT,neiljay/RIOT,luciotorre/RIOT,khhhh/RIOT,kaspar030/RIOT,dailab/RIOT,sgso/RIOT,jfischer-phytec-iot/RIOT,rakendrathapa/RIOT,watr-li/RIOT,BytesGalore/RIOT,katezilla/RIOT,cladmi/RIOT,gebart/RIOT,abp719/RIOT,ximus/RIOT,kushalsingh007/RIOT,brettswann/RIOT,jremmert-phytec-iot/RIOT,patkan/RIOT,miri64/RIOT,roberthartung/RIOT,ThanhVic/RIOT,asanka-code/RIOT,OlegHahm/RIOT,Lexandro92/RIOT-CoAP,l3nko/RIOT,malosek/RIOT,patkan/RIOT,chris-wood/RIOT,ant9000/RIOT,Osblouf/RIOT,MohmadAyman/RIOT,adrianghc/RIOT,herrfz/RIOT-old,bartfaizoltan/RIOT,MohmadAyman/RIOT,thiagohd/RIOT,1blankz7/RIOT,miri64/RIOT,avmelnikoff/RIOT,Ell-i/RIOT,mziegert/RIOT,LudwigOrtmann/RIOT,robixnai/RIOT,koenning/RIOT,bartfaizoltan/RIOT,latsku/RIOT,smlng/RIOT,OTAkeys/RIOT,herrfz/RIOT-old,watr-li/RIOT,jferreir/RIOT,A-Paul/RIOT,hamilton-mote/RIOT-OS,alex1818/RIOT,Darredevil/RIOT,DipSwitch/RIOT,msolters/RIOT,binarylemon/RIOT,watr-li/RIOT