Log Message:
Update ruby-RMagick package to 2.6.0.

RMagick 2.6.0
    o Fix bug #21237, Image::write ignores format attribute when called with a
      Tempfile pathname (bug report by Jack Shedd)
    o Fix bug #21897, ImageList#from_blob abends when certain corrupt JPEG
      images are used (bug report by Peter Szabo)
    o Add Image#composite_tiled, Image#composite_tiled! (ref:
      http://rubyforge.org/forum/forum.php?thread_id=27347&forum_id=33)
    o Add Image#deskew (available with ImageMagick 6.4.2-5)
    o Add Image#define, Image#undefine (available in ImageMagick 6.3.6)
    o Add Image#level_colors (available in ImageMagick 6.4.2-1)
    o Add Image#levelize_channel (available in ImageMagick 6.4.2-1)
    o Add Image#affinity, ImageList#affinity (available in ImageMagick 6.4.3-6).
      These methods replace Image#map and ImageList#map.
    o Accept DitherMethod values for the dither argument to Image#quantize,
      ImageList#quantize
    o Add the BarrelDistortion, PolynomialDistortion, ShepardsDistortion,
      PolarDistortion, and DePolarDistortion MagickDistortion Method enum values
      (available in ImageMagick 6.4.2-6)
    o Add the HorizontalTileVirtualPixelMethod and
      VerticalTileVirtualPixelMethod VirtualPixelMethod enum values
      (available in ImageMagick 6.4.2-6)
    o Add DitherMethod enum class
    o Added general-purpose OptionalMethodArguments class to handle ad-hoc
      optional arguments.
    o Support optional "distort:viewport" and \ 
"distort:scale" arguments to
      Image#distort
    o Support optional highlight_color and lowlight_color arguments
      to Image#compare_channel

RMagick 2.5.2
    o Add support for MergeLayer to Magick::ImageList#optimize_layers (patch
      #21189, submitted by Andrew Watson)
    o Add PowQuantumOperator argument for Image#quantum_operator (available
      in ImageMagick 6.4.1-9)

RMagick 2.5.1
    o Update Pixel#to_color to work with the new QueryMagickColorname API in
      ImageMagick 6.4.1-9.

RMagick 2.5.0
    o Added Image#add_compose_mask, #delete_compose_mask (feature #20531)

RMagick 2.4.0
    o Added Image#image_type= (feature #20490)