Fixed import order in core/models.py

wagtail/wagtailcore/models.py

@@ -14,7 +14,7 @@ from django.core.handlers.base import BaseHandler
 from django.core.handlers.wsgi import WSGIRequest
 from django.core.urlresolvers import reverse
 from django.db import connection, models, transaction
-from django.db.models import Q, Case, When, IntegerField
+from django.db.models import Case, IntegerField, Q, When
 from django.db.models.signals import post_delete, post_save, pre_delete
 from django.dispatch.dispatcher import receiver
 from django.http import Http404