Bati minha cabeça na parede por um tempo até encontrar essa dica no Stack Overflow.
O erro
Can't install RMagick 2.13.1. Can't find the ImageMagick library or one of the dependent libraries. Check the mkmf.log file for more detailed information.
O conserto
brew install imagemagick
cd /usr/local/Cellar/imagemagick/6.8.0-10/lib
ln -s libMagick++-Q16.7.dylib libMagick++.dylib
ln -s libMagickCore-Q16.7.dylib libMagickCore.dylib
ln -s libMagickWand-Q16.7.dylib libMagickWand.dylib
gem install rmagick