importlib does not exist on Python 2.6. Use Django version.

mongoengine/django/mongo_auth/models.py

@@ -1,9 +1,8 @@
-from importlib import import_module
-
 from django.conf import settings
 from django.contrib.auth.models import UserManager
 from django.core.exceptions import ImproperlyConfigured
 from django.db import models
+from django.utils.importlib import import_module
 from django.utils.translation import ugettext_lazy as _