CI: Specify the Rubinius version more tightly.
CI: Specify the Rubinius version more tightly.

And allow them to fail. Rubinius builds are kind of broken on Travis CI.

libgit2/rugged,saraid/rugged,libgit2/rugged,Acidburn0zzz/rugged,Acidburn0zzz/rugged,Acidburn0zzz/rugged,libgit2/rugged,tpickett66/rugged,saraid/rugged,tpickett66/rugged,tpickett66/rugged,saraid/rugged