Raise error if user tries to use Rabl but they haven't included it in their Gemfile.

Author: Bushjumper

lib/gon.rb

@@ -60,6 +60,9 @@ def set_variable(name, value)
     end
 
     def rabl(view_path, options = {})
+      if !defined?(Gon::Rabl)
+        raise NoMethodError.new('You should define Rabl in your Gemfile') 
+      end
       rabl_data = Gon::Rabl.parse_rabl(view_path, options[:controller] || 
                                                   @request_env['action_controller.instance'] || 
                                                   @request_env['action_controller.rescue.response'].