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.
Merge pull request #127 from 3flex/remove-rubygems-2.2.0-workaround

remove workaround for Rubygems 2.2.0 bug