Version update check

We do a http HEAD request to determine if update notifications are available for the current locale. If there is no update notification available, we then fallback to en locale for the GET request.

This is part of the effort to get rid of download.brackets.io. Currently the webserver will handle the url rewrite for requests that look for an update notification in a specific locale.

The server doesn't have to do anything here anymore, since we handle this in Brackets.