Watch for changes on all branches.
Allow failures on rbx builds.

Rubinius has been getting unstable againlately , and we've seen
multiple travis builds failing. Putting Rubinius back in the
allow_failures list will make silence those build failures for the time
being.
Fix the travis build for Rubinius.
Merge pull request #268 from libgit2/arthur/travis-changes

Changes to the Travis build configuration