CI: Build Rugged on OS X.
CI: Bump the ruby 2.1.x version.

But allow failures when building with 2.1.2 on OS X, as that seems to be broken.
CI: Specify the Rubinius version more tightly.

And allow them to fail. Rubinius builds are kind of broken on Travis CI.
Merge pull request #376 from libgit2/arthur/multi-os-builds

Multi OS Builds