buildroot

package/imagemagick: change download url to github

Upstream quickly removes old versions from
http://www.imagemagick.org/download/releases

For our LTS versions we should switch to a stable upstream repo which
provides all released versions.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>