[Xastir] Don't upgrade to ImageMagick 5.5.8

[Dick C. Reichenbach]

This isn't going to surprise anyone on this list, but the latest and
greatest from Imagemagick breaks something is Xastir.

After spending a month or so unable to recompile Xastir, I've finally
regressed back to IM 5.5.7 and everything seems back to normal. 
Developers: Don't worry, I don't plan to go back to 5.5.8, so I wouldn't
worry about it.

Thanks 

Dick Reichenbach - KC8OBZ