imagemagick: use official download site

Don't use anymore the alternate download site beacause it does not
conatins anymore older versions, instead use the official download site
because now it contains even the older versions.

Signed-off-by: Fabio Porcedda <Fabio.Porcedda@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>