Try the new travis-ci build environment

See the blog post: http://blog.travis-ci.com/2014-12-17-faster-builds-with-container-based-infrastructure/

Other projects seem to have decreased build time tremendously. We can't use sudo with this but afaik we don't need to.

There is also bundler caching support now, but I can't see how we can invalidate the cache without a `Gemfile.lock`: http://docs.travis-ci.com/user/caching/