travis: Use Docker-based VMs
travis: Use Docker-based VMs

They are faster and more reliable.

tyduptyler13/ejs,kolonse/ejs,cnwhy/ejs,TimothyGu/ejs-tj,mde/ejs,mmis1000/ejs-promise,operatino/ejs,TimothyGu/ejs-tj,insidewarehouse/ejs,xanxiver/ejs,jtsay362/solveforall-ejs2,rpaterson/ejs,insidewarehouse/ejs,kolonse/ejs,cnwhy/ejs,zensh/ejs,zensh/ejs