Workaround to build against Rubinius

> When using Rubinius, there's currently an issue with selecting the
> correct version in RVM in our build environment, but only when
> specifying rbx as your version. As a workaround, specify rbx-2
> instead

http://docs.travis-ci.com/user/languages/ruby/#Choosing-Ruby-versions-and-implementations-to-test-against