Run rbx builds also in 2.0 mode.
Run rbx builds also in 2.0 mode.

And stop allowing failures on rbx. We haven't had a single build failure on rubinius for quite some time now.

Also removed an unused line of the Travis Matrix that I added for cross-compilation.
libgit2/rugged,Acidburn0zzz/rugged,saraid/rugged,tpickett66/rugged,libgit2/rugged,tpickett66/rugged,tpickett66/rugged,Acidburn0zzz/rugged,saraid/rugged,libgit2/rugged,Acidburn0zzz/rugged,saraid/rugged