Maybe fix gc cleanup error (Closes celery/celery/#993)
Maybe fix gc cleanup error (Closes celery/celery/#993)

flaviogrossi/billiard,jakirkham/billiard,flaviogrossi/billiard,jakirkham/billiard,pombredanne/billiard,djsmith42/billiard,andresriancho/billiard,djsmith42/billiard,andresriancho/billiard,pombredanne/billiard