added rubinius 2.3.0 (rbx-2.3.0)

I added the support for Rubinius 2.3.0 (rbx-2.3.0) using LLVM 3.4
Tested on:

* Ubuntu 15.04 [Vivid Vervet (development branch)]
* rbenv 0.4.0
* ruby-build 20140926


The fix for OS X would be to remove this line. It only has effect on OS X anyway and is needed for previous rbx releases.