Support HTTP access to /feed.xml

The same issue that was described at https://github.com/Polymer/blog/issues/7 is affecting the RSS feed for chromestatus.com.

I'm all in favor of HTTPS-everywhere, but having an RSS feed that Feedly can't deal with is pretty much equivalent to not having an RSS feed.

HSTS will take care of redirecting to the HTTPS version of the feed, so this should have minimal impact on end users with user agents that are modern browsers.