Reverting core_signing to the old way.

By reverting django.core.signing to Django 1.11.8 version Django-Q works
This is not designed as a 'good' solution just to show where I think the
problem is.

django_q/signing.py

@@ -4,7 +4,7 @@ try:
 except ImportError:
     import pickle
 
-from django.core import signing
+from django_q import core_signing as signing
 
 from django_q.conf import Conf