Comment 8 for bug 1562560

Version number of dependency should be removed as well (IM 7.x came out in 2011!) as this prevents the replacement of IM with GM via graphicsmagick-imagemagick-compat

I attempted to submit a bug for that, but launchpad timed out and lost my essay-sized report :|

It may not be appropriate for this to be changed upstream, as Debian has different support timelines than Ubuntu.

TL;DR replace "imagemagick (>= 6.4~)" with "imagemagick"