add missing `skip` to a ruby grade-school test file

Unlike the other ruby tests I've come across, once the required ruby file is present, the grade-school test starts by raising eight errors instead of just one. Then, before the `test_an_empty_school` assertion passes, it'll complain about missing methods `add` and `grade` because the `test_get_students_sorted_in_a_grade` assertion isn't being skipped.