Upgrade Django to 1.8.15 with Zulip patches.

requirements/common.txt

@@ -1,6 +1,6 @@
 -r ipython.txt
 # Django itself; we use a slightly patched version
-git+https://github.com/zulip/truncated-django.git
+git+https://github.com/zulip/truncated-django-1.8.15.git@cbf4fa3aef1b17f37d75a70e57f9b69a0f99ed5c#egg=Django==1.8.15
 
 GitPython==0.3.2.1
 

tools/travis/success-http-headers.txt

@@ -24,7 +24,6 @@ Reusing existing connection to localhost:443.
   Content-Type: text/html; charset=utf-8
   Transfer-Encoding: chunked
   Connection: keep-alive
-  Cache-Control: max-age=0
   Strict-Transport-Security: max-age=15768000
 Length: unspecified [text/html]
 Saving to: ‘/tmp/index.html’