remove workaround for Rubygems 2.2.0 bug

https://github.com/rubygems/rubygems/pull/763 is merged and released in Rubygems 2.2.1. A forced downgrade is no longer required.

Commit: https://github.com/rubygems/rubygems/commit/335ea9d79989fced8f02225115c89d7a8534fa49