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

This closes https://github.com/travis-ci/travis-ci/issues/2952
and provides a workaround for https://github.com/travis-ci/travis-ci/issues/2821
Merge pull request #346 from travis-ci/ha-bug-rbx-bundler-args

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