ObligationScope should use references in Rails 4 when it uses includes.

Before Rails 4, includes would also outer join tables if query conditions referenced those tables.  Rails 4 has dropped this behavior in favor of making the developer explicitly identify includes that must also be joined with an additional references() method on the scope.  @zeiv modified tests to use references(), but it seems better to me to incorporate this directly into the scope() method of ObligationScope.  This PR reverts the Rails 4-specific tests and instead sets references in Rails >= 4.