Fix enqueue at with queue validation

enqueue_at_with_queue currently doesn't work in 1.9.10 as validate_job! tries to pull the queue name off the class, which fails if your class doesn't define a default queue (as it may not if you are using an explicit queue in your call to enqueue_at_with_queue). First commit adds a failing test (also locks the version of rufus-scheduler to < 3.0 in the gemspec, otherwise tests won't run). Second commit fixes it!