pkg-download: update the github helper

Once again, github updates the source download url.
Even if only the zip archive link is advertised on the repositories'
page, the *.tar.gz is still available.

It is worthy to note that the tarball's content differs depending if
it has been fetched from the former and the new url (the root directory
name changes).

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>