Inline controller's permitted_attributes method

Fixes thoughtbot#417

Problem:

If users include the Pundit gem on an Administrate controller,
the controller attempts to call the `permitted_attributes` method
defined by pundit instedad of the one defined by Administrate.

Because these methods take different arguments,
this results in an `ArgumentError` for the user.

Solution:

Inline the `permitted_attributes` method
to avoid the method name collision.

This narrows the public API a bit, but that's alright.
The user can accomplish all of the removed functionality by overriding
the parent `resource_params` method.

app/controllers/administrate/application_controller.rb

@@ -100,11 +100,7 @@ def find_resource(param)
     end
 
     def resource_params
-      params.require(resource_name).permit(*permitted_attributes)
-    end
-
-    def permitted_attributes
-      dashboard.permitted_attributes
+      params.require(resource_name).permit(dashboard.permitted_attributes)
     end
 
     delegate :resource_class, :resource_name, :namespace, to: :resource_resolver