Require yaml for isolation test

It was removed when we removed mocha at
5a6ae7f753

activemodel/test/cases/serializers/xml_serialization_test.rb

@@ -2,6 +2,7 @@ require 'cases/helper'
 require 'models/contact'
 require 'active_support/core_ext/object/instance_variables'
 require 'ostruct'
+require 'yaml'
 
 module Admin
   class Contact < ::Contact