Speed up travis build by using the xenial container instead of the trusty VM
Speed up travis build by using the xenial container instead of the trusty VM

Older versions of firefox crashed when used in a docker container, which is why we used sudo:required to get a full VM, it appears that this is no longer an issue

Also split out the linting to a seperate build stage for a bit of parallelism and cache the .deb download of firefox (saves about 1.5 minutes)

computestdev/Openrunner,computestdev/Openrunner