Disable parallel bundle installs causing failures on rbx

https://github.com/travis-ci/travis-ci/issues/2821
https://github.com/mvz/rubocop/commit/345a90957c2f16861356d887e23733a43f85ca56
Merge pull request #12 from brandondunne/disable_concurrent_bundle_install

Disable parallel bundle installs causing failures on rbx