Force single-job bundle install

rubygems has threadsafety issues which rubinius exposes; this can lead
to corrupted gems during `bundle install`. See:

https://github.com/rubygems/rubygems/issues/982
https://github.com/travis-ci/travis-ci/issues/2821
https://github.com/mvz/rubocop/commit/345a90957c2f16861356d887e23733a43f85ca56