Added gem runtime dependency on coffee-script.

The "require 'coffee_script'" in lib/spree_auth_devise.rb implies that
coffee-script is a runtime dependency, not just a development
dependency. Apps that include Spree but don't explicitly declare
coffee-script as a dependency currently fail when they reach that line.

I don't actually know if '>= 2.2.0' is correct, but I'm guessing it is because
that is the version number used by coffee-rails 4.0.0, which is listed as a 
development dependency.