Cache bundler

Especially important with Rubinius since it'll
download all of rubysl.
Relax rbx version requirement

Run the latest 2.x
No longer need :rbx platform group

See
http://www.benjaminfleischer.com/2013/12/05/testing-rubinius-on-travis-ci/
Merge pull request #73 from calagator/rbx

Fix CI build to test on Rubinius