imagemagick: use alternate download location

The official ImageMagick site only keeps the latest versions. Even in
the legacy/ directory, they only keep the last version of each
'branch'. Therefore, to avoid problems, we use an alternate download
site.

Fixes
http://autobuild.buildroot.org/results/2269aa42a0a21730ff0ec28af89cd4973ec28751/build-end.log

.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>