Rename of ParseMagickOption() in Imagemagick 6.8.5

Hi Jeremy,

this pull request is only to keep track of this breaking change in ImageMagick 6.8.5 (at least). I didn't go back to check which version renamed it, but the changeset is the following: http://trac.imagemagick.org/changeset/4281/ImageMagick/trunk/magick/image.c

`ParseMagickOption()` was renamed to `ParseCommandOption()`.

This was the only change I needed to do to compile my mod_dims with ImageMagick 8.6.5-10 (latest as of this writing).

I'm currently testing it.