I forget this every time I run Rspec on a new Debian. Maybe if I write it down it will help me in the future.

Selenium::WebDriver::Error::UnknownError:
       unknown error: Chrome failed to start: exited abnormally
         (unknown error: DevToolsActivePort file doesn't exist)
         (The process started from chrome location /usr/bin/google-chrome is no longer running, so ChromeDriver is assuming that Chrome has crashed.)

Solution:

export DISPLAY=:99
/usr/bin/Xvfb -ac $DISPLAY -screen 0 1024x768x16