It appears it’s a problem reported on the Homebrew github repo (https://github.com/mxcl/homebrew/issues/16625) blaming rmagick itself not supporting newer versions of imagemagick. On that same issue (https://github.com/mxcl/homebrew/issues/16625#issuecomment-11519383), you can find this link: https://coderwall.com/p/wnomjg which actually worked for me. This is what he does:
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