[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/lib/bugdown/testing_mocks.py

@@ -1,10 +1,10 @@
 
-import simplejson
+import ujson
 
 def twitter(tweet_id):
     if tweet_id not in ["112652479837110273", "287977969287315456", "287977969287315457"]:
         return None
-    return simplejson.loads("""{
+    return ujson.loads("""{
   "coordinates": null,
   "created_at": "Sat Sep 10 22:23:38 +0000 2011",
   "truncated": false,