Fix travis builds for rbx
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/
roadtrippers/acts_as_list,swanandp/acts_as_list,pacoguzman/acts_as_list,amitree/acts_as_list,udit7590/acts_as_list,tinderfields/acts_as_list,joeadcock/acts_as_list,corwinstephen/acts_as_list,ludwigschubert/acts_as_list,botandrose/acts_as_list,exAspArk/acts_as_list,fabn/acts_as_list,qLearning/acts_as_list