Upgrade to Django 1.11

Seems almost incredible but no code update was needed. Lucky us !

… Or careless me, we will see ;-).

requirements/base.txt

@@ -1,4 +1,4 @@
-Django>=1.9.3,<1.10
+Django>=1.11,<2
 PyYAML>=3.11,<4.0
 django-request-token>=0.6,<0.7
 pytz