selenium-server-standalone と chromedriver をインストール
$ brew update
$ brew install selenium-server-standalone
$ brew install chromedriver
参考
https://gist.github.com/stoshiya/3dc36187ede3e6eee3e4
エラーが出た場合
$ brew install chromedriver
Error: No available formula with the name "chromedriver"
Found a cask named "chromedriver" instead.
$ brew cask install chromedriver
==> Satisfying dependencies
==> Downloading https://chromedriver.storage.googleapis.com/75.0.3770.8/chromedriver_mac64.zip
######################################################################## 100.0%
==> Verifying SHA-256 checksum for Cask 'chromedriver'.
==> Installing Cask chromedriver
==> Linking Binary 'chromedriver' to '/usr/local/bin/chromedriver'.
🍺 chromedriver was successfully installed!