use -ping for more efficient IM identify

identify with -format will get killed when attempted with higher resolution images. -ping will extract the image metadata with much less resource use.

lib/iiif_print/image_tool.rb

@@ -74,7 +74,7 @@ def im_identify_geometry(lines)
 
     # @return [Array<String>] lines of output from imagemagick `identify`
     def im_identify
-      cmd = "identify -format 'Geometry: %G\nDepth: %[bit-depth]\nColorspace: %[colorspace]\nAlpha: %A\nMIME type: %m\n' #{path}"
+      cmd = "identify -ping -format 'Geometry: %G\nDepth: %[bit-depth]\nColorspace: %[colorspace]\nAlpha: %A\nMIME type: %m\n' #{path}"
       `#{cmd}`.lines
     end