Log Message:
Update ruby-RMagick to 2.11.0.

Changes:
RMagick 2.11.0
    o Fix bug #26475, dissolve and watermark don't work with new versions of
      ImageMagick (reported by Jim Crate)
    o Add Image#composite_mathematics (available in ImageMagick 6.5.4-3)
    o Add new LinearDodgeCompositeOp, LinearBurnCompositeOp,
      PegtopCompositeOp, PinLightCompositeOp, VividLightCompositeOp enum values
      (available in ImageMagick 6.5.4-3)