This is mostly an issue due to changes in imagemagick 6.8, which is now what is installed by homebrew by default. This is the procedure I used to get rmagick installed. Rmagick 2.13.2 has some changes for 6.8 compatibility which help fix the extension compilation, but I also found additional issues with libtool dylibs not existing for the specified architecture.
brew uninstall imagemagick
brew install imagemagick
brew install libtool --universal
brew link libtool --force
gem install rmagick --version=2.13.2
The re-install of imagemagick may not be necessary, but I found the forced linking of libtool necessary for imagemagick versions 6.8+