Use Travis CIs new container based builds.

The announcement is here: http://blog.travis-ci.com/2014-12-17-faster-builds-with-container-based-infrastructure/
The specs here: http://docs.travis-ci.com/user/workers/container-based-infrastructure/

This should provide throughput benefits immediately, but even if a wash, it sets the stage for leveraging caching for
quicker and more stable builds: http://docs.travis-ci.com/user/caching/

Testing Done:
CI went green quickly here: https://travis-ci.org/pantsbuild/pants/builds/44617837

Bugs closed: 881

Reviewed at https://rbcommons.com/s/twitter/r/1523/