Traceback on some github messages

Here's what I see in the logs:

```
   File "/usr/lib/python2.7/site-packages/github2fedmsg/views/webhooks.py", line 132, in webhook
     fas_usernames = build_fas_lookup(payload)
   File "/usr/lib/python2.7/site-packages/github2fedmsg/views/webhooks.py", line 172, in build_fas_lookup
     usernames.add(commit['committer']['username'])
```