Upgrade Django to 1.8.16.

Fixes #3906

See more details about this security release here:
https://www.djangoproject.com/weblog/2016/nov/01/security-releases/

requirements/prod.txt

@@ -4,9 +4,9 @@ Babel==2.3.4 \
     --hash=sha256:3318ed2960240d61cbc6558858ee00c10eed77a6508c4d1ed8e6f7f48399c975 \
     --hash=sha256:c535c4403802f6eb38173cd4863e419e2274921a01a8aad8a5b497c131c62875
 # Django is required by django-aesfield, django-cache-nuggets, django-csp, django-mozilla-product-details, django-tables2, nobot
-Django==1.8.15 \
-    --hash=sha256:e2e41aeb4fb757575021621dc28fceb9ad137879ae0b854067f1726d9a772807 \
-    --hash=sha256:863e543ac985d5cfbce09213fa30bc7c802cbdf60d6db8b5f9dab41e1341eacd
+Django==1.8.16 \
+    --hash=sha256:cc3a95187788627dfdc94b41de908aadfc4241fabb3ceaef19f4bd3b89c0fdf7 \
+    --hash=sha256:224aaf17a28609707d942deafe6d0a5b382baf22a6f33e4e61c56c62f09081dd
 GitPython==0.1.7 \
     --hash=sha256:03754bc7b256397c1b646e5048e2291590f5080171adb8b00f4e2d7384c76eee
 MySQL-python==1.2.5 \