Log Message:
Update RMagick to 2.5.2.

Changes:
RMagick 2.5.2
    o Add support for MergeLayer to Magick::ImageList#optimize_layers (patch
      #21189, submitted by Andrew Watson)
    o Add PowQuantumOperator argument for Image#quantum_operator (available
      in ImageMagick 6.4.1-9)