Renamed deprecated celeryd to celery worker.

Running celeryd, the script stores this in mediagoblin-celeryd.pid :
``
("\nThe 'celeryd' command is deprecated, please use 'celery worker' instead:\n\n$ celery worker --pidfile=/var/run/mediagoblin/mediagoblin-celeryd.pid -f /var/log/mediagoblin/mediagoblin-celeryd.log\n\n",)
``
The proposed changes fix that issue.