status/web/feeds.py: fix pubDate format

A prior localization fix caused RFC822-style dates to be rendered as follows:

1, 23 7 2013 13:04:18 GMT

This implements the fix proposed in http://trac.buildbot.net/ticket/2530

This is the same patch as in https://github.com/buildbot/buildbot/pull/806 (feeds.py is currently the same in both master and buildbot-0.8.8)