# Install old homebrew package

brew uninstall ImageMagick
gem uninstall rmagick

# Second, please see https://github.com/rmagick/rmagick/issues/256 and read the answer by hristozov, viz:
brew rm imagemagick
cd "$(brew --prefix)/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/"
git checkout 6f014f2b7f1f9e618fd5c0ae9c93befea671f8be imagemagick.rb
brew install imagemagick
brew pin imagemagick
git reset --hard

# This will force homebrew to use /usr/local/Cellar/imagemagick/6.9.7-3

# Third,

gem install rmagick