Fixed deprecation warning in admin migration

wagtail/wagtailadmin/migrations/0001_create_admin_access_permissions.py

@@ -1,6 +1,7 @@
 # -*- coding: utf-8 -*-
 from __future__ import unicode_literals
 
+import django
 from django.db import migrations
 
 
@@ -11,11 +12,18 @@ def create_admin_access_permissions(apps, schema_editor):
 
     # Add a fake content type to hang the 'can access Wagtail admin' permission off.
     # The fact that this doesn't correspond to an actual defined model shouldn't matter, I hope...
-    wagtailadmin_content_type = ContentType.objects.create(
-        app_label='wagtailadmin',
-        model='admin',
-        name='Wagtail admin'
-    )
+    if django.VERSION >= (1, 8):
+        wagtailadmin_content_type = ContentType.objects.create(
+            app_label='wagtailadmin',
+            model='admin'
+        )
+    else:
+        # Django 1.7 and below require a content type name
+        wagtailadmin_content_type = ContentType.objects.create(
+            app_label='wagtailadmin',
+            model='admin',
+            name='Wagtail admin'
+        )
 
     # Create admin permission
     admin_permission = Permission.objects.create(