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",)

docker-test/api/requirements.txt

@@ -81,7 +81,7 @@ jsonschema==2.6.0
 keyring==18.0.0
 keystoneauth1==3.8.0
 kitchen==1.2.6
-kombu==4.6.5
+kombu==4.6.3
 linecache2==1.0.0
 lxml==4.2.1
 MarkupSafe==1.0