Bug 893514 - Remove "en-US" and update SUMO links in /firefox/update

1. We shouldn't have en-US hard-coded in links to let the locale
redirection work on SUMO
2. Links are out of date, so we end up having 3 redirects and broken
anchors even on en-US
Merge pull request #1064 from flodolo/bug893514update

Bug 893514 - Remove "en-US" and update SUMO links in /firefox/update