Newsflash: Vitaly Slobodin, the PhantomJS maintainer, is stepping down, since Google Chrome version 59 will ship with its own headless option. This means you’re going to have to move all your capybara tests from PhantomJS into ChromeDriver.
Then you’ll have to install the latest
brew install chromedriver
Then register selenium-webdriver as your capybara driver, with chrome as the browser.
chromeOptions: check the headless chrome documentation.
disable-gpu is temporarily required to run headless, but it may not be necessary in the future.