Use correct get_user_model

That bug was introduced in 869944d and caused tests to fail with Django
1.4.

oscar/apps/customer/mixins.py

@@ -1,10 +1,10 @@
 from django.conf import settings
-from django.contrib.auth import (authenticate, get_user_model,
-                                 login as auth_login)
+from django.contrib.auth import authenticate, login as auth_login
 from django.contrib.sites.models import get_current_site
 from django.db.models import get_model
 from oscar.apps.customer.signals import user_registered
 from oscar.core.loading import get_class
+from oscar.core.compat import get_user_model
 
 User = get_user_model()
 CommunicationEventType = get_model('customer', 'CommunicationEventType')