Alias 'rbx' to 'rbx-weekly-d18'

This was requested in travis-ci/travis-build#89

From that issue:

> This is the most similar given the old defaults on Travis (ie, Rubinius
> defaults to 1.8 mode and Travis was updated every few weeks with a build
> of the Rubinius 2.0.testing branch).

/cc @joshk