aqbanking: Add an updater script

The promised updater, which feeds sources.nix and should help updating
all the aqbanking libraries at the same time.

I was very reluctant to write the updater entirely in Nix as I've done a
while ago for Chromium, because this would involve ugly hacks to do so.

That's why I've done a more conservative approach here, only using
nix-instantiate to get the contents of the current sources.nix.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @cillianderoiste, @urkud