Update ruby18-RMagick pacakge to 2.7.0.

RMagick 2.7.0
    o Fix bug #22152, extconf.rb does not respect the LDFLAGS environment
      variable (bug report by Joseph Sokol-Margolis)
    o Fix bug #22190, the NoDitherMethod enum value is not defined in
      ImageMagick 6.4.2
    o Add the TrimBoundsLayer ImageLayerMethod enum value (available in
      ImageMagick 6.4.3-8)
    o Add the CopyAlphaChannel, ExtractAlphaChannel, OpaqueAlphaChannel,
      ShapeAlphaChannel, and TransparentAlphaChannel AlphaChannelType enum
      values (available in ImageMagick 6.4.3-7)
    o Rename Image#affinity and ImageList#affinity to Image#remap and
      ImageList#remap. Retain the old names as aliases. (Changed in ImageMagick
      6.4.4-0)

(taca)