[manual] Use ujson instead of simplejson.

This saves something like 15ms on our 1000 message get_old_messages
queries, and will save even more when we start sending JSON dumps into
our memcached system.

We need to install python-ujson on servers and dev instances before
pushing this to prod.

(imported from commit 373690b7c056d00d2299a7588a33f025104bfbca)

zephyr/management/commands/subscribe_new_users.py

@@ -1,6 +1,6 @@
 from __future__ import absolute_import
 
-import simplejson
+import ujson
 from postmonkey import PostMonkey
 from django.core.management.base import BaseCommand
 from django.conf import settings