After brewing:
cd /usr/local/Cellar/imagemagick/6.8.8-9/lib
ln -s libMagick++-6.Q16.3.dylib libMagick++.dylib
ln -s libMagickCore-6.Q16.2.dylib libMagickCore.dylib
ln -s libMagickWand-6.Q16.2.dylib libMagickWand.dylib
Modified from http://stackoverflow.com/questions/13942443/error-installing-rmagick-on-mountain-lion/13960185#13960185