Log Message:
Update ruby-RMagick to 1.15.0

ChangeLog:
RMagick 1.15.0
    o Added fx method to ImageList class
    o Added wet_floor method to the Image class
    o Added linear_stretch method to the Image class (available with
      ImageMagick 6.3.1-1)
    o Added recolor method to the Image class (available with ImageMagick 6.3.1-
3)
    o Added polaroid method to the Image class (available with ImageMagick 6.3.1
-6)
    o Added origin attribute to the Image::Info class (supported by
      ImageMagick 6.3.1-4 and later)
    o Added PaletteBilevelMatteType to the ImageType enum (available with
      ImageMagick 6.2.9)
    o Fix bug #6260, some RVG examples produce all-black GIF images
    o Fix bug #7034, fix the matte method in the Draw class
    o Fix bug #7373, default channels should be RGB instead of RGBA
    o Fix bug #7716, Pixel#intensity wrong for gray images (bug report from
      Morio Miki)
    o Fix bug #7949, Magick::Draw.new abends when an exception occurs before
      the draw object is fully initialized (bug report from Andrew Kaspick)
    o Fix bug #8015, Magick::Draw.new doesn't call the optional arguments block
      in the right scope (bug report from Andrew Kaspick)
    o Tested with ImageMagick 6.3.2-0