Update RMagick to 2.1.0.

Changes:
RMagick 2.1.0
    o Added Image::Info#caption= attribute
    o Rename Image#crop_resized, #crop_resized! to #resize_to_fill,
      #resize_to_fill!. Add aliases for the old names.
    o Fix bug #16776, in the axes.rb example the last 2 arguments to
      border.rectangle are swapped (bug report by Alain Feler)
    o Fix bug #16931, apostrophe in #error directive causes error in some
      versions of GCC (bug report by Justin Dossey)

(obache)