Fix deprecation of Travi's rbx-18/19modes

http://docs.travis-ci.com/user/languages/ruby/#Rubinius

> Note that the syntax of rbx-19mode isn't supported anymore.

**Update**: Rubinius is still whacky. The [Twitter Gem allows failures](https://github.com/sferik/twitter/blob/85a6eb0701c19c261195ce637748434855df28ac/.travis.yml#L19), so I propose to go with the flow.