Update travis build.
Update travis build.

- Use bundle caching (see http://blog.travis-ci.com/2014-12-17-faster-builds-with-container-based-infrastructure/)
- Add ruby 2.2.
- Allow it to fail for now; it’s not out yet and
  has some failures.
rspec/rspec-dev,rspec/rspec-dev,rspec/rspec-dev