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.
It fetches the latest version based on the bucketlist XML from
commondatastorage and generates a "source.nix" which contains an attribute set
about where to fetch the latest version.
The XML is parsed in a somewhat hackish way using sed, but as this is just an
updater, its okay and we don't want to break a fly on the wheel by employing a
full XML parser.
