Various .travis.yml improvements

1. Use `language` key to explicitly indicate Ruby. (While Ruby is the default on Travis CI, this may change in the future.)
1. Send the builds to container-based infrastructure, so that…
1. We can cache gems to speed up builds (`~/.phantomjs` is also included, but I believe something is cleaning up that directory before `rspec`).
1. Run builds on Rubinius, but allow it to fail.

Additionally, we pin RSpec to 2.14.x.