allow depends_on to be a job id or a job itself

Other parts of the code (i.e.: the `.create()` method) allow the `depends_on` kwarg to be a `Job` object *or* a job id. This is an attempt to allow that same idea within the `.enqueue_call()` method for a queue. Since this part of the code requires actually knowing the precise redis key for the job that's depended on, my intuition is that a `.fetch()` is the only way to solve this.

rq/queue.py

@@ -189,6 +189,8 @@ class Queue(object):
         # If WatchError is raised in the process, that means something else is
         # modifying the dependency. In this case we simply retry
         if depends_on is not None:
+            if not isinstance(depends_on, Job):
+                depends_on = Job.fetch(id=depends_on, connection=self.connection)
             with self.connection.pipeline() as pipe:
                 while True:
                     try: