Lock redis-py to 2.10.6 #921

This is a stopgap for a compatibility issue relating to a
dependency of a dependency (redis-py, which is a requirement of
django-redis). See:
jazzband/django-redis#342

config/requirements_core.txt

@@ -26,3 +26,4 @@ channels_redis==2.2.1                   # Channels Layer
 
 # Misc
 ipython==6.3.1
+redis==2.10.6