Log Message:
Update graphics/ruby-RMagick to 2.13.1.

* Use lang/ruby/gem.mk instead of misc/rubygems/rubygem.mk.
* Remove default value of GEM_BUILD.

RMagick 2.13.1
    o Fixed bug preventing RMagick from working with version 6.5.9 or higher

RMagick 2.13.0
    o Added Doxygen documentation, for automatic documentation
    o Fixed bug #27467, get RMagick to compile witH ImageMagick 6.5.7
    o Fixed bug #27607, switch Pixel#from_hsla and Pixel#to_hsla to use ranges
      0-255 instead of 0-100 for saturation and lightness (range used by
      ImageMagick 6.5.6-5 and higher). Also added ability to specify all
      arguments to these functions as percentages (bug report by Arthur Chan).