Don't protect attributes under Rails 5

The protected_attributes gem is not compatible for Rails 5, and we don't
need it anyway with Strong Parameters.

spec/support/unit/model_creation_strategies/active_record.rb

@@ -42,7 +42,7 @@ def define_class_for_model
           run_block(model, block)
         end
 
-        if whitelist_attributes?
+        if whitelist_attributes? && model.respond_to?(:attr_accessible)
           model.attr_accessible(*columns.keys)
         end