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

Patch applied and tested on 0.8.8rc1. Dates now render properly with Vienna.app, and http://feedvalidator.org only has an unrelated recommendation.