Update .travis.yml
Update .travis.yml

I re-enabled Rubinius and set `allow_failures` per the docs here http://about.travis-ci.org/docs/user/build-configuration/. Hopefully I did that correctly.

It seems that allowing failures would be better than adding and removing Rubinius. That way, when it fails, it's easy to point us to a failure.
travis-ci/travis-api,Tiger66639/travis-api,travis-ci/travis-api,Tiger66639/travis-api,travis-ci/travis-api