Bump rmagick to 2.13.3.

rmagick-2.13.2 is incompatible with versions of ImageMagick built with --enable-hdri. This includes the version of ImageMagick supplied with Arch Linux. This prevents rmagick, and consequently Danbooru,
from being installed on Arch Linux.

rmagick-2.13.3 fixed this issue (https://github.com/rmagick/rmagick/issues/18)