Adding missing import.

modoboa/core/models.py

@@ -5,6 +5,7 @@ import logging
 import reversion
 from django.db import models
 from django.db.models.signals import post_delete
+from django.conf import settings
 from django.dispatch import receiver
 from django.utils import timezone
 from django.utils.translation import ugettext as _, ugettext_lazy