alsamixergui: fix download URL

Closes #3457

The Debian package snapshot service moved to an official debian.org
address last year (http://www.debian.org/News/2010/20100412

) with a
slightly different URL structure, so adjust the download URL to match.

Signed-off-by: Hector Oron <hector.oron@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>