Windows binaries for ImageMagick 6.7.9.9 are no longer available

[ikarth]

As referenced in #89 the ImageMagick 6.7 Windows binaries are no longer available. The solution there no longer works, as the ftp server that the installation instructions reference archived their old content in 2016, but did not include the ImageMagick binaries, possibly because ImageMagick is still under active development and they assumed that legacy binaries would still be available from the project itself.

There may other locations for the binaries, but a cursory search didn't turn up any.