Downgrade kombu from 4.6.5 to 4.6.3

As suggested in celery/kombu#1063
This should fixes the issue with redis key getting evicted every now and then
and should mean we stop receiving the following error:
Control command error: OperationalError(u"\nCannot route message for exchange u'reply.celery.pidbox': Table empty or key no longer exists.\nProbably the key (u'_kombu.binding.reply.celery.pidbox') has been removed from the Redis database.\n",)

requirements/base.txt

@@ -13,6 +13,8 @@ requests==2.19.1
 uwsgi==2.0.17.1
 xlrd==0.9.3
 django-moj-irat>=0.4
+# https://github.com/celery/kombu/issues/1063
+kombu==4.6.3
 
 # locks for Python 2.7
 more-itertools==5.0.0