Log Message:
Update RMagick to 2.2.0.
While here, add DESTDIR support.

Changes:
RMagick 2.2.0
    o Added Image#opaque_channel, Image#paint_transparent (available with
      ImageMagick 6.3.7-10)
    o Added Image#liquid_rescale (available with ImageMagick 6.3.8-2)
    o Added CMYColorspace ColorspaceType value
    o Fixed bug #17148, compiler error message on Solaris (bug report by Trever
      Wennblom)
    o Fixed bug #17470, get_exif_by_number, get_exif_by_entry may fail when
      called with one or more arguments