I upgraded to imagemagick-7.0.4-3.sierra and now my previously installed RMagck 2.16 fails. I checked for MagicWand:
$ find /usr/local -name MagickWand.h
/usr
dyson's solution didn't quite work for me. I found that in addition to installing imagemagick@6, I also need to install pkg-config and then set PKG_CONFIG_PATH.
brew install imagemagick@6 pkg-config
PATH="/usr/local/opt/imagemagick@6/bin:$PATH" PKG_CONFIG_PATH=/usr/local/opt/imagemagick@6/lib/pkgconfig gem install rmagick -v '2.13.4'