nginx: Fix accidental load-balancing between IPv4 and IPv6.

Apparently, our nginx configuration's use of "localhost", combined
with the default in modern Linux of having localhost resolve to both
the IPv4 and IPv6 addresses on a given machine, resulted in `nginx`
load-balancing requests to a given Zulip server between the IPv4 and
IPv6 addresses.  This, in turn, resulted in irrelevant 502 errors
problems every few minutes on the /events endpoints for some clients.

Disabling IPv6 on the server resolved the problem, as does simply
spelling localhost as 127.0.0.1 for the `nginx` upstreams that we
declare for proxying to non-Django services on localhost.

puppet/zulip/files/nginx/zulip-include-frontend/upstreams

@@ -3,14 +3,14 @@ upstream django {
 }
 
 upstream tornado {
-    server localhost:9993;
+    server 127.0.0.1:9993;
     keepalive 10000;
 }
 
 upstream localhost_sso {
-    server localhost:8888;
+    server 127.0.0.1:8888;
 }
 
 upstream camo {
-    server localhost:9292;
+    server 127.0.0.1:9292;
 }