Tests fail with Rspec 3, so pin at 2.14.x.
Switch to rbx-2 for Travis build.

This should fix the "Requested binary installation but no rubies are available
to download" error in Travis for rbx.
Merge pull request #1075 from mbrictson/pin-rspec-2.14.x

Fix Travis build by pinning Rspec at 2.14.x and switching to rbx-2