Fix travis builds for rbx

Currently, the Travis build is failing due to a wrong specified version of
rbx. According to the docs, Travis currently expects rbx-2 here.

> 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.

-- from http://docs.travis-ci.com/user/languages/ruby/