Bug 916229 - proposing a patch to fix the bug.

Ah, please use `cache.set` here.

`cache.add` will only add the value to the key if there is no such key already. So imagine we'd run this task every hour and add the value with a lifetime of 24 hours. After 24 hours the key is invalidated and effectively "not there" anymore for Memcache. If for some reason a small queue builds up and the scheduling of the task for the hour *after* those 24 hours is a bit too late it won't populate correctly and leave the cache unpopulated till the next approximatly one hour later. So `cache.set` is the better choice as it re-sets the value every hour and makes sure that we increase the window in which a Celery or Memcache failure can happen is 24 hours.