Log Message:
Update RMagick to 2.8.0.

Changes:
RMagick 2.8.0
    o Add the endian, scene, and transparent_color attributes to Image::Info
    o Deprecate Image#endian= attribute setter
    o Add the transparent_chroma method to the Image class (available in
      ImageMagick 6.4.5-6)
    o Add the sparse_color method to the Image class (available in ImageMagick
      6.4.3)
    o Detect change to ParseSizeGeometry signature in ImageMagick 6.4.6-9 and
      compile accordingly.
    o Update Image#change_geometry to work with the new ParseSizeGeometry API
      in ImageMagick 6.4.6-9.