In Mac OSX Sierra, to solve the issue, I had to do the following:
Clean:
brew uninstall imagemagick
brew unlink imagemagick
Then install:
brew install pkgconfig
brew install imagemagick@6
brew link imagemagick@6 --force
Install gem rmagick
PKG_CONFIG_PATH=/usr/local/opt/imagemagick@6/lib/pkgconfig gem install rmagick