Revert "Require rainbow gem version to be 2.1.x due to sickill/rainbow#40 (#516)" (#520)
Revert "Require rainbow gem version to be 2.1.x due to sickill/rainbow#40 (#516)" (#520)

This reverts commit 11ac0bb4a4578e33261db6c76202b4f6fdeb1f97.
exercism/xruby,Insti/exercism-ruby,Insti/exercism-ruby,NeimadTL/ruby,tommyschaefer/xruby,tommyschaefer/xruby,Insti/exercism-ruby,Insti/xruby,NeimadTL/ruby,exercism/xruby,NeimadTL/ruby,Insti/xruby