The previos update script just used the last version of chromium that showed up
at the bucket list at:
http://commondatastorage.googleapis.com/chromium-browser-official/
I'm not sure which channel this list actually holds, so I'm going to switch now
using the official release channels grabbed by omahaproxy. This also has the
advantage that we can provide different versions/flavors of chromium.
We now also write our data to sources.nix instead of source.nix, as we have more
than one source.