ActiveJob: Implemented enqueue_at for QueueClassic

Also we currently support only queue_classic < 3 so I've added those condition to the Gemfile
For the normal tests that would work because the tests expects enqueue_at to run the job inline immediately. For integration tests we are testing that jobs scheduled in the future don't run.
Ok. I'll remove this and we'll see what we will do in the integration tests when we get there