Ruby 2.2 on Rails 4.0 support

In Ruby 2.2 Minitest has been removed from ruby core. However, Rails 4.0.x includes it as a gem dependency. In order to not fail on loading our adapter this performs a second check for Ruby 2.2+ which then handles the Minitest 4.x gem.

Resolve #1262 
Is there case where gem is not defined ?