rmagickがgem installできない「Can't install RMagick 2.15.4. Can't find MagickWand.h.」

• MacOS EL Capitan 10.11.6
• Ruby 2.3.3

Macをクリーンインストールして、会社のプロジェクトを環境構築した時のお話...

$ bundle install

Installing therubyracer 0.12.2 with native extensions
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.
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checking for clang... yes
checking for Magick-config... no
checking for pkg-config... yes
checking for outdated ImageMagick version (<= 6.4.9)... no
checking for Ruby version >= 1.8.5... yes
checking for stdint.h... yes
checking for sys/types.h... yes
checking for wand/MagickWand.h... no

Can't install RMagick 2.15.4. Can't find MagickWand.h.
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Provided configuration options:
    --with-opt-dir
    --without-opt-dir
    --with-opt-include
    --without-opt-include=${opt-dir}/include
    --with-opt-lib
    --without-opt-lib=${opt-dir}/lib
    --with-make-prog
    --without-make-prog
    --srcdir=.
    --curdir
    --ruby=/Users/taku/.rbenv/versions/2.3.3/bin/$(RUBY_BASE_NAME)

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An error occurred while installing rmagick (2.15.4), and Bundler cannot continue.
Make sure that `gem install rmagick -v '2.15.4'` succeeds before bundling.

問題は、rmagick さんが ImageMagick 7系に対応してないんですって。
imagemagickのバージョン確認は、こちら。

$ brew info imagemagick
imagemagick: stable 7.0.5-3 (bottled), HEAD

解決方法

手順1

まずは、いったん削除する

$ brew uninstall imagemagick

手順2

6系を再インストール

brew install https://raw.githubusercontent.com/Homebrew/homebrew-core/6f014f2b7f1f9e618fd5c0ae9c93befea671f8be/Formula/imagemagick.rb

手順3

もっかいrmagicをインストールする

$ gem install rmagick