vagrant: update ubuntu version in readme
vagrant: update ubuntu version in readme

syin2/RIOT,ks156/RIOT,TobiasFredersdorf/RIOT,rfuentess/RIOT,alignan/RIOT,OlegHahm/RIOT,neumodisch/RIOT,jbeyerstedt/RIOT-OTA-update,kYc0o/RIOT,miri64/RIOT,OTAkeys/RIOT,kaleb-himes/RIOT,kaleb-himes/RIOT,lazytech-org/RIOT,rfuentess/RIOT,kerneltask/RIOT,BytesGalore/RIOT,adrianghc/RIOT,authmillenon/RIOT,Josar/RIOT,ks156/RIOT,tfar/RIOT,LudwigOrtmann/RIOT,TobiasFredersdorf/RIOT,toonst/RIOT,x3ro/RIOT,dkm/RIOT,Hyungsin/RIOT-OS,miri64/RIOT,cladmi/RIOT,x3ro/RIOT,neumodisch/RIOT,plushvoxel/RIOT,dkm/RIOT,RIOT-OS/RIOT,avmelnikoff/RIOT,jbeyerstedt/RIOT-OTA-update,rfuentess/RIOT,dkm/RIOT,alignan/RIOT,LudwigOrtmann/RIOT,hamilton-mote/RIOT-OS,LudwigOrtmann/RIOT,aeneby/RIOT,jfischer-phytec-iot/RIOT,d00616/RIOT,LudwigKnuepfer/RIOT,roberthartung/RIOT,neiljay/RIOT,A-Paul/RIOT,syin2/RIOT,authmillenon/RIOT,lazytech-org/RIOT,A-Paul/RIOT,TobiasFredersdorf/RIOT,dailab/RIOT,basilfx/RIOT,kbumsik/RIOT,kaspar030/RIOT,authmillenon/RIOT,yogo1212/RIOT,biboc/RIOT,ks156/RIOT,kbumsik/RIOT,mfrey/RIOT,plushvoxel/RIOT,Ell-i/RIOT,neumodisch/RIOT,BytesGalore/RIOT,Ell-i/RIOT,kYc0o/RIOT,mtausig/RIOT,kYc0o/RIOT,d00616/RIOT,lazytech-org/RIOT,adrianghc/RIOT,ks156/RIOT,basilfx/RIOT,d00616/RIOT,smlng/RIOT,kaspar030/RIOT,alignan/RIOT,rfuentess/RIOT,kb2ma/RIOT,katezilla/RIOT,kbumsik/RIOT,Ell-i/RIOT,cladmi/RIOT,dailab/RIOT,thomaseichinger/RIOT,OlegHahm/RIOT,jasonatran/RIOT,miri64/RIOT,ant9000/RIOT,biboc/RIOT,LudwigOrtmann/RIOT,hamilton-mote/RIOT-OS,miri64/RIOT,tfar/RIOT,backenklee/RIOT,yogo1212/RIOT,mtausig/RIOT,neiljay/RIOT,syin2/RIOT,tfar/RIOT,Josar/RIOT,kaleb-himes/RIOT,kbumsik/RIOT,kaspar030/RIOT,smlng/RIOT,katezilla/RIOT,yogo1212/RIOT,LudwigKnuepfer/RIOT,mtausig/RIOT,avmelnikoff/RIOT,alignan/RIOT,katezilla/RIOT,toonst/RIOT,Hyungsin/RIOT-OS,yogo1212/RIOT,BytesGalore/RIOT,kb2ma/RIOT,josephnoir/RIOT,authmillenon/RIOT,kb2ma/RIOT,rfuentess/RIOT,mtausig/RIOT,ant9000/RIOT,jbeyerstedt/RIOT-OTA-update,mfrey/RIOT,cladmi/RIOT,d00616/RIOT,Hyungsin/RIOT-OS,kYc0o/RIOT,BytesGalore/RIOT,backenklee/RIOT,adjih/RIOT,beurdouche/RIOT,beurdouche/RIOT,gautric/RIOT,roberthartung/RIOT,backenklee/RIOT,thomaseichinger/RIOT,Ell-i/RIOT,d00616/RIOT,OTAkeys/RIOT,dailab/RIOT,hamilton-mote/RIOT-OS,jasonatran/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,lazytech-org/RIOT,lazytech-org/RIOT,backenklee/RIOT,aeneby/RIOT,dkm/RIOT,kb2ma/RIOT,immesys/RiSyn,plushvoxel/RIOT,roberthartung/RIOT,miri64/RIOT,kbumsik/RIOT,tfar/RIOT,basilfx/RIOT,RIOT-OS/RIOT,biboc/RIOT,OlegHahm/RIOT,beurdouche/RIOT,avmelnikoff/RIOT,OlegHahm/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,gebart/RIOT,hamilton-mote/RIOT-OS,neumodisch/RIOT,cladmi/RIOT,kaleb-himes/RIOT,katezilla/RIOT,adjih/RIOT,smlng/RIOT,yogo1212/RIOT,kerneltask/RIOT,syin2/RIOT,thomaseichinger/RIOT,TobiasFredersdorf/RIOT,x3ro/RIOT,tfar/RIOT,backenklee/RIOT,kerneltask/RIOT,jfischer-phytec-iot/RIOT,hamilton-mote/RIOT-OS,LudwigOrtmann/RIOT,jfischer-phytec-iot/RIOT,ant9000/RIOT,josephnoir/RIOT,avmelnikoff/RIOT,kerneltask/RIOT,gautric/RIOT,neiljay/RIOT,beurdouche/RIOT,ant9000/RIOT,mfrey/RIOT,A-Paul/RIOT,mfrey/RIOT,jbeyerstedt/RIOT-OTA-update,Ell-i/RIOT,OTAkeys/RIOT,adrianghc/RIOT,TobiasFredersdorf/RIOT,Josar/RIOT,d00616/RIOT,roberthartung/RIOT,immesys/RiSyn,biboc/RIOT,smlng/RIOT,jbeyerstedt/RIOT-OTA-update,smlng/RIOT,gautric/RIOT,thomaseichinger/RIOT,kb2ma/RIOT,jasonatran/RIOT,jasonatran/RIOT,aeneby/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,gebart/RIOT,neumodisch/RIOT,Josar/RIOT,A-Paul/RIOT,Josar/RIOT,aeneby/RIOT,toonst/RIOT,kaleb-himes/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,jasonatran/RIOT,basilfx/RIOT,ant9000/RIOT,katezilla/RIOT,OTAkeys/RIOT,LudwigKnuepfer/RIOT,adrianghc/RIOT,toonst/RIOT,A-Paul/RIOT,LudwigOrtmann/RIOT,gautric/RIOT,mfrey/RIOT,immesys/RiSyn,foss-for-synopsys-dwc-arc-processors/RIOT,immesys/RiSyn,Hyungsin/RIOT-OS,alignan/RIOT,josephnoir/RIOT,RIOT-OS/RIOT,kerneltask/RIOT,RIOT-OS/RIOT,basilfx/RIOT,jfischer-phytec-iot/RIOT,neiljay/RIOT,josephnoir/RIOT,LudwigKnuepfer/RIOT,thomaseichinger/RIOT,mtausig/RIOT,avmelnikoff/RIOT,x3ro/RIOT,adjih/RIOT,aeneby/RIOT,gebart/RIOT,x3ro/RIOT,dailab/RIOT,immesys/RiSyn,ks156/RIOT,kaspar030/RIOT,adjih/RIOT,toonst/RIOT,BytesGalore/RIOT,kaspar030/RIOT,adjih/RIOT,Hyungsin/RIOT-OS,gautric/RIOT,authmillenon/RIOT,adrianghc/RIOT,jfischer-phytec-iot/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,dkm/RIOT,authmillenon/RIOT,biboc/RIOT,neumodisch/RIOT,OlegHahm/RIOT,josephnoir/RIOT,plushvoxel/RIOT,syin2/RIOT,OTAkeys/RIOT,beurdouche/RIOT,LudwigKnuepfer/RIOT,gebart/RIOT,neiljay/RIOT,immesys/RiSyn,gebart/RIOT,plushvoxel/RIOT,roberthartung/RIOT,yogo1212/RIOT,dailab/RIOT,cladmi/RIOT