Use Travis containers for faster tests

See:
http://blog.travis-ci.com/2014-12-17-faster-builds-with-container-based-infrastructure/
Use bundler cache on Travis

The Travis container infrastructure supports bundler caching for open
source builds. For projects with a Gemfile.lock, this can significantly
speed up builds. In our case it may not result in as big of a speedup
due to the need to install the gems for our various appraisals.