Only run Travis on code changes. (#4868)
Only run Travis on code changes. (#4868)

This prevents the Travis run when only markdown files are changed, or
only examples/spec files are changed. Aka, only run Travis on code
changes.

Inspiration https://github.com/facebook/react/commit/233af7e2dff4c79619f5cda6943388ff9e21581b.
kmh287/amphtml,zhucl/amphtml,lzanol/amphtml,tdrl/amphtml,ADITION/amphtml,ruturajv-media-net/amphtml,laterpay/amphtml,ericlindley-g/amphtml,honeybadgerdontcare/amphtml,sortable/amphtml,jridgewell/amphtml,laterpay/amphtml,adup-tech/amphtml,jridgewell/amphtml,mhc-gh/amphtml,Mobusi-ads/amphtml,gumgum/amphtml,media-net/amphtml,camelburrito/amphtml,DAC-f1h-dev/amphtml,sklobovskaya/amphtml,adtechnology/amphtml,vmarteev/amphtml,shawnbuso/amphtml,Adtoma/amphtml,dotandads/amphtml,BenDz/amphtml,taboola/amphtml,smartadserver/amphtml,ecoron/amphtml,dotandads/amphtml,kmh287/amphtml,taboola/amphtml,avimehta/amphtml,mhc-gh/amphtml,shawnbuso/amphtml,avimehta/amphtml,devjsun/amphtml,tlong2/amphtml,laterpay/amphtml,voyagegroup/amphtml,koshka/amphtml,yieldmo/amphtml,engtat/amphtml,gumgum/amphtml,fullspeed-sp/amphtml,jpettitt/amphtml,sriramkrish85/amphtml,rifler/amphtml,avimehta/amphtml,ecoron/amphtml,fullspeed-sp/amphtml,kamtschatka/amphtml,engtat/amphtml,src-code/amphtml,spotxchange/amphtml,comScore/amphtml,kixer-com/amphtml,Mobusi-ads/amphtml,jridgewell/amphtml,widespace-os/amphtml,tdrl/amphtml,zhangsu/amphtml,yieldmo/amphtml,DarXector/amphtml,donttrustthisbot/amphtml,zhouyx/amphtml,techhtml/amphtml,dvoytenko/amphtml,BenDz/amphtml,adup-tech/amphtml,cory-work/amphtml,mlb7687/amphtml,aghassemi/amphtml,wjfang/amphtml,prateekbh/amphtml,media-net/amphtml,DAC-f1h-dev/amphtml,yieldmo/amphtml,rifler/amphtml,danielrozenberg/amphtml,Gregable/amphtml,burtcorp/amphtml,Lith/amphtml,voyagegroup/amphtml,sebastianbenz/amphtml,Gregable/amphtml,Mobusi-ads/amphtml,alanorozco/amphtml,sharethrough/amphtml,zhucl/amphtml,lannka/amphtml,aghassemi/amphtml,alanorozco/amphtml,lannka/amphtml,Lith/amphtml,yieldbot/amphtml,voyagegroup/amphtml,rifler/amphtml,yandex-pcode/amphtml,interspacejp/amphtml,spotxchange/amphtml,Adtoma/amphtml,rsimha-amp/amphtml,luciancrasovan/amphtml,ADITION/amphtml,sriramkrish85/amphtml,yandex-pcode/amphtml,lzanol/amphtml,jridgewell/amphtml,chaveznvg/amphtml,jmadler/amphtml,burtcorp/amphtml,kamtschatka/amphtml,clawr/amphtml,rlugojr/amphtml,clawr/amphtml,ecoron/amphtml,yieldmo/amphtml,jmadler/amphtml,donttrustthisbot/amphtml,alanorozco/amphtml,adup-tech/amphtml,erwinmombay/amphtml,burtcorp/amphtml,pietergreyling/amphtml,jdelhommeau/amphtml,fullspeed-sp/amphtml,Gregable/amphtml,widespace-os/amphtml,clawr/amphtml,Lith/amphtml,keithwrightbos/amphtml,adtechnology/amphtml,dvoytenko/amphtml,bpaduch/amphtml,cvializ/amphtml,pietergreyling/amphtml,ADITION/amphtml,ruturajv-media-net/amphtml,yieldmo/amphtml,DAC-f1h-dev/amphtml,aghassemi/amphtml,gumgum/amphtml,shawnbuso/amphtml,DAC-f1h-dev/amphtml,DarXector/amphtml,Mixpo/amphtml,jpettitt/amphtml,cory-work/amphtml,yandex-pcode/amphtml,danielrozenberg/amphtml,ericlindley-g/amphtml,lzanol/amphtml,adup-tech/amphtml,yandex-pcode/amphtml,spotxchange/amphtml,yieldbot/amphtml,techhtml/amphtml,alanorozco/amphtml,honeybadgerdontcare/amphtml,mlb7687/amphtml,src-code/amphtml,taboola/amphtml,keithwrightbos/amphtml,sklobovskaya/amphtml,triplelift/amphtml,honeybadgerdontcare/amphtml,media-net/amphtml,Adtoma/amphtml,adiant/amphtml,rlugojr/amphtml,widespace-os/amphtml,engtat/amphtml,interspacejp/amphtml,cory-work/amphtml,Lith/amphtml,adiant/amphtml,Mixpo/amphtml,wjfang/amphtml,sharethrough/amphtml,dvoytenko/amphtml,BenDz/amphtml,gumgum/amphtml,bpaduch/amphtml,erwinmombay/amphtml,JuliusThms/amphtml,dotandads/amphtml,luciancrasovan/amphtml,lzanol/amphtml,vmarteev/amphtml,nexxtv/amphtml,comScore/amphtml,sriramkrish85/amphtml,powdercloud/amphtml,Gregable/amphtml,donttrustthisbot/amphtml,devjsun/amphtml,pedjoni8/amphtml,Gregable/amphtml,wjfang/amphtml,media-net/amphtml,ericlindley-g/amphtml,arealit/amphtml,Lith/amphtml,ruturajv-media-net/amphtml,zhouyx/amphtml,adtechnology/amphtml,vmarteev/amphtml,devjsun/amphtml,taboola/amphtml,rlugojr/amphtml,ericlindley-g/amphtml,dotandads/amphtml,DarXector/amphtml,sortable/amphtml,honeybadgerdontcare/amphtml,powdercloud/amphtml,Mobusi-ads/amphtml,jridgewell/amphtml,ampproject/amphtml,rifler/amphtml,lannka/amphtml,JuliusThms/amphtml,rifler/amphtml,yieldbot/amphtml,arealit/amphtml,zhangsu/amphtml,Mixpo/amphtml,gumgum/amphtml,mlb7687/amphtml,jdelhommeau/amphtml,techhtml/amphtml,ADITION/amphtml,sriramkrish85/amphtml,JuliusThms/amphtml,sklobovskaya/amphtml,danielrozenberg/amphtml,kamtschatka/amphtml,DistroScale/amphtml,avimehta/amphtml,lzanol/amphtml,clawr/amphtml,comScore/amphtml,yieldbot/amphtml,adup-tech/amphtml,fullspeed-sp/amphtml,media-net/amphtml,taboola/amphtml,adtechnology/amphtml,aghassemi/amphtml,DAC-f1h-dev/amphtml,ampproject/amphtml,clawr/amphtml,interspacejp/amphtml,fullspeed-sp/amphtml,sebastianbenz/amphtml,Mobusi-ads/amphtml,chaveznvg/amphtml,triplelift/amphtml,taboola/amphtml,ruturajv-media-net/amphtml,koshka/amphtml,cvializ/amphtml,voyagegroup/amphtml,Adtoma/amphtml,mhc-gh/amphtml,sharethrough/amphtml,JuliusThms/amphtml,devjsun/amphtml,mlb7687/amphtml,arealit/amphtml,sebastianbenz/amphtml,mhc-gh/amphtml,jridgewell/amphtml,luciancrasovan/amphtml,DistroScale/amphtml,sharethrough/amphtml,jridgewell/amphtml,Adtoma/amphtml,vmarteev/amphtml,sklobovskaya/amphtml,ampproject/amphtml,yandex-pcode/amphtml,lannka/amphtml,zhouyx/amphtml,cvializ/amphtml,src-code/amphtml,zhucl/amphtml,yandex-pcode/amphtml,sriramkrish85/amphtml,shawnbuso/amphtml,techhtml/amphtml,luciancrasovan/amphtml,sriramkrish85/amphtml,rajkumarsrk/amphtml,camelburrito/amphtml,avimehta/amphtml,kixer-com/amphtml,donttrustthisbot/amphtml,smartadserver/amphtml,rlugojr/amphtml,rajkumarsrk/amphtml,DarXector/amphtml,aghassemi/amphtml,arealit/amphtml,mhc-gh/amphtml,yieldbot/amphtml,rajkumarsrk/amphtml,prateekbh/amphtml,camelburrito/amphtml,nexxtv/amphtml,keithwrightbos/amphtml,zhucl/amphtml,adiant/amphtml,nexxtv/amphtml,avimehta/amphtml,koshka/amphtml,ecoron/amphtml,widespace-os/amphtml,camelburrito/amphtml,keithwrightbos/amphtml,jlucero-contentad/amphtml,sebastianbenz/amphtml,sebastianbenz/amphtml,jpettitt/amphtml,bpaduch/amphtml,ericlindley-g/amphtml,alanorozco/amphtml,chaveznvg/amphtml,jlucero-contentad/amphtml,jpettitt/amphtml,honeybadgerdontcare/amphtml,sebastianbenz/amphtml,DistroScale/amphtml,lannka/amphtml,prateekbh/amphtml,ruturajv-media-net/amphtml,adup-tech/amphtml,nexxtv/amphtml,DarXector/amphtml,adiant/amphtml,interspacejp/amphtml,donttrustthisbot/amphtml,ampproject/amphtml,jlucero-contentad/amphtml,rsimha-amp/amphtml,ecoron/amphtml,triplelift/amphtml,jpettitt/amphtml,mlb7687/amphtml,cory-work/amphtml,alanorozco/amphtml,pietergreyling/amphtml,keithwrightbos/amphtml,gumgum/amphtml,src-code/amphtml,koshka/amphtml,luciancrasovan/amphtml,burtcorp/amphtml,rsimha-amp/amphtml,triplelift/amphtml,mhc-gh/amphtml,comScore/amphtml,prateekbh/amphtml,burtcorp/amphtml,adiant/amphtml,kmh287/amphtml,nexxtv/amphtml,kamtschatka/amphtml,sharethrough/amphtml,lzanol/amphtml,laterpay/amphtml,rajkumarsrk/amphtml,lannka/amphtml,rsimha-amp/amphtml,BrightcoveOS/amphtml,prateekbh/amphtml,vmarteev/amphtml,Mixpo/amphtml,bpaduch/amphtml,zhangsu/amphtml,comScore/amphtml,tlong2/amphtml,DistroScale/amphtml,ampproject/amphtml,bpaduch/amphtml,nexxtv/amphtml,sortable/amphtml,pietergreyling/amphtml,arealit/amphtml,voyagegroup/amphtml,triplelift/amphtml,fullspeed-sp/amphtml,ampproject/amphtml,spotxchange/amphtml,wjfang/amphtml,voyagegroup/amphtml,adiant/amphtml,koshka/amphtml,JuliusThms/amphtml,keithwrightbos/amphtml,alanorozco/amphtml,pedjoni8/amphtml,smartadserver/amphtml,BrightcoveOS/amphtml,mlb7687/amphtml,wjfang/amphtml,keithwrightbos/amphtml,prateekbh/amphtml,voyagegroup/amphtml,koshka/amphtml,cvializ/amphtml,jmadler/amphtml,shawnbuso/amphtml,vmarteev/amphtml,zhouyx/amphtml,Mixpo/amphtml,donttrustthisbot/amphtml,sklobovskaya/amphtml,adtechnology/amphtml,DarXector/amphtml,powdercloud/amphtml,wjfang/amphtml,rifler/amphtml,src-code/amphtml,kixer-com/amphtml,rajkumarsrk/amphtml,zhouyx/amphtml,zhangsu/amphtml,cvializ/amphtml,pedjoni8/amphtml,DAC-f1h-dev/amphtml,arealit/amphtml,engtat/amphtml,devjsun/amphtml,BenDz/amphtml,BrightcoveOS/amphtml,dotandads/amphtml,luciancrasovan/amphtml,laterpay/amphtml,lannka/amphtml,danielrozenberg/amphtml,widespace-os/amphtml,aghassemi/amphtml,DistroScale/amphtml,Mobusi-ads/amphtml,sortable/amphtml,zhouyx/amphtml,erwinmombay/amphtml,pedjoni8/amphtml,jpettitt/amphtml,tdrl/amphtml,jpettitt/amphtml,laterpay/amphtml,BrightcoveOS/amphtml,honeybadgerdontcare/amphtml,shawnbuso/amphtml,media-net/amphtml,DistroScale/amphtml,techhtml/amphtml,rajkumarsrk/amphtml,nexxtv/amphtml,media-net/amphtml,danielrozenberg/amphtml,yieldmo/amphtml,tdrl/amphtml,rlugojr/amphtml,adtechnology/amphtml,rsimha-amp/amphtml,rajkumarsrk/amphtml,kamtschatka/amphtml,jdelhommeau/amphtml,tlong2/amphtml,adiant/amphtml,ampproject/amphtml,powdercloud/amphtml,jdelhommeau/amphtml,mhc-gh/amphtml,honeybadgerdontcare/amphtml,kixer-com/amphtml,sharethrough/amphtml,dvoytenko/amphtml,danielrozenberg/amphtml,alanorozco/amphtml,yieldbot/amphtml,Mixpo/amphtml,spotxchange/amphtml,ruturajv-media-net/amphtml,prateekbh/amphtml,BrightcoveOS/amphtml,ecoron/amphtml,camelburrito/amphtml,jmadler/amphtml,arealit/amphtml,cory-work/amphtml,camelburrito/amphtml,ADITION/amphtml,jmadler/amphtml,kixer-com/amphtml,Gregable/amphtml,chaveznvg/amphtml,erwinmombay/amphtml,spotxchange/amphtml,sklobovskaya/amphtml,smartadserver/amphtml,ampproject/amphtml,sklobovskaya/amphtml,erwinmombay/amphtml,techhtml/amphtml,sortable/amphtml,gumgum/amphtml,tlong2/amphtml,wjfang/amphtml,jdelhommeau/amphtml,src-code/amphtml,Lith/amphtml,zhangsu/amphtml,tlong2/amphtml,interspacejp/amphtml,taboola/amphtml,jpettitt/amphtml,kmh287/amphtml,kamtschatka/amphtml,JuliusThms/amphtml,comScore/amphtml,jdelhommeau/amphtml,rsimha-amp/amphtml,clawr/amphtml,Gregable/amphtml,zhucl/amphtml,devjsun/amphtml,pietergreyling/amphtml,jlucero-contentad/amphtml,donttrustthisbot/amphtml,powdercloud/amphtml,shawnbuso/amphtml,zhucl/amphtml,smartadserver/amphtml,rsimha-amp/amphtml,ericlindley-g/amphtml,engtat/amphtml,zhouyx/amphtml,camelburrito/amphtml,dvoytenko/amphtml,widespace-os/amphtml,interspacejp/amphtml,triplelift/amphtml,chaveznvg/amphtml,engtat/amphtml,cory-work/amphtml,zhucl/amphtml,kmh287/amphtml,engtat/amphtml,tlong2/amphtml,vmarteev/amphtml,chaveznvg/amphtml,pedjoni8/amphtml,rlugojr/amphtml,luciancrasovan/amphtml,erwinmombay/amphtml,danielrozenberg/amphtml,dvoytenko/amphtml,BenDz/amphtml,kmh287/amphtml,yandex-pcode/amphtml,BenDz/amphtml,zhangsu/amphtml,cvializ/amphtml,dotandads/amphtml,yieldmo/amphtml,jlucero-contentad/amphtml,burtcorp/amphtml,jmadler/amphtml,media-net/amphtml,bpaduch/amphtml,pedjoni8/amphtml,smartadserver/amphtml,kixer-com/amphtml,comScore/amphtml,mhc-gh/amphtml,bpaduch/amphtml,cory-work/amphtml,tdrl/amphtml,Adtoma/amphtml,pietergreyling/amphtml,powdercloud/amphtml,jlucero-contentad/amphtml,tdrl/amphtml,sortable/amphtml,powdercloud/amphtml,clawr/amphtml,danielrozenberg/amphtml,Gregable/amphtml,BrightcoveOS/amphtml,kixer-com/amphtml