chromium/updater: Fix getting latest versions

Comparing the current version with the version in sources list and
accidentally swapping the version arguments isn't going to get very far
because every new version that will come up will then be treated as "we
already have that version".

So we're now using versionOlder and also a check whether the version is
the *same* as the one in sources.nix.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>