Support Ruby 2.2.0

Ah right. But rspec-rails depend on test-unit.
https://github.com/rspec/rspec-rails/blob/v2.14.0/lib/rspec/rails/adapters.rb#L3

The latest version 3.1.0 also depend on it. https://github.com/rspec/rspec-rails/blob/v3.1.0/lib/rspec/rails/adapters.rb#L19