Revert "Drop '--jobs' from bundler args when running on Rubinius"

This reverts commit b64eea75aba4a42187aad8bff0be3e1702d379b5.

Rubinius 2.4.1 has bee released
(https://github.com/travis-ci/travis-ci/issues/2952#issuecomment-65842593)
and we would like to get some additional data if that release
is good for addressing the original problem.