duma: fix downloading by forcing extension to .tar.gz
Downoading a non-existing file from sourceforge gives you a "200 OK"
and an index.html. As we try to retrieve a .tar.bz2 first, and duma
is bundled in a .tar.gz, we won't get appropriate content, so
just force the extension to avoid the problem.
Thanks to Ingmar Schraub <is@eseco.de> for pointing out the issue.