requirements: add version specifier to django-guardian

django-guardian drops support for django==1.4 so this specifier is
required to ensure django-userena works with django==1.4.
Note that django guardian 2.0 will drop support for this release
of django.

setup.py

@@ -11,7 +11,12 @@
         "``long_description`` (%s)\n" % readme_file)
     sys.exit(1)
 
-install_requires = ['easy_thumbnails', 'django-guardian', 'html2text==2014.12.29']
+install_requires = [
+    'easy_thumbnails',
+    'django-guardian<=1.3.1',
+    'html2text==2014.12.29'
+]
+
 try:
     from collections import OrderedDict
 except ImportError: