Log Message:
Update rmagick to 2.12.2.

RMagick 2.12.2
    o Add feature tests for SinusoidFunction and PolynomialFunction enum
      values to allow compiling with ImageMagick 6.4.8-7 (bug report by Mark
      Richman)

RMagick 2.12.1
    o Fix bug #27239, allow 2.12.0 to compile with older releases of ImageMagick
      (bug report by Sam Lown)

RMagick 2.12.0
    o Added Image#function_channel (available in ImageMagick 6.4.8-8)
    o Added Image#auto_level_channel, Image#auto_gamma_channel (available in
      ImageMagick 6.5.5-1)
    o Added Draw#interline_spacing, #interline_spacing= (available in
      ImageMagick 6.5.5-8)