Travis: Use container-based platform

[timretout]

See: http://blog.travis-ci.com/2014-12-17-faster-builds-with-container-based-infrastructure/

It's faster!

[moollaza]

@diocles thanks a lot -- we actually attempted this previosly but we need to wait because it creates problems as the .travis.yml file in the root of the repo gets overwritten at build time by the TravisCI Dist::Zilla Plugin.

See #1456