Get rubinius builds running on Travis CI

See https://github.com/travis-ci/travis-ci/issues/1641#issuecomment-29773392
Cache installed gems on Travis CI builds

http://about.travis-ci.org/docs/user/caching/
Need to specify ruby standard library gems for rbx

see http://www.benjaminfleischer.com/2013/12/05/testing-rubinius-on-travis-ci/
Add more rubinius-specific gems
Travis CI: Let rubinius fail; enable fast finish when rbx fails

refs
http://about.travis-ci.org/docs/user/build-configuration/
http://about.travis-ci.org/blog/2013-11-27-fast-finishing-builds/
Merge pull request #180 from bf4/fix_build

thanks, merging