Log Message:
update RMagick to 2.3.0.

Changes:
o Added Image#encipher, Image#decipher (available with ImageMagick 6.3.8-6)
o Added DTX1Compression, DTX1Compression, and DTX1Compression
  CompressionType enums (available in ImageMagick 6.3.9-4)
o Added optional "use hex format" argument to Pixel#to_color
o Support :area resource type in Magick.limit_resource
o Pixel.from_HSL and Pixel#to_HSL are deprecated. Use Pixel.from_hsla
  and Pixel#to_hsla instead. The new methods rely on the ImageMagick 6.3.5
  and later API.
o The Image#alpha and alpha= attributes are deprecated. Use alpha() and
   alpha? instead.
o The Image#mask= attribute is deprecated. Use mask() instead.
o The use of Ruby older than version 1.8.4 with RMagick is deprecated and
  will not be supported in a future release.
o Fix bug #18271, rvg width and height attributes wrong after a call to
  viewbox (reported by Greg Jarman)