Fixed #17627 -- Renamed util.py files to utils.py

Thanks PaulM for the suggestion and Luke Granger-Brown and
Wiktor Kołodziej for the initial patch.

tests/admin_custom_urls/tests.py

@@ -1,7 +1,7 @@
 from __future__ import unicode_literals
 import warnings
 
-from django.contrib.admin.util import quote
+from django.contrib.admin.utils import quote
 from django.core.urlresolvers import reverse
 from django.template.response import TemplateResponse
 from django.test import TestCase