Xvfb not found on your system (Headless :: Exception) on mac

/Users/ct/.rvm/gems/ruby-2.1.1/gems/headless-1.0.2/lib/headless/cli_util.rb:9:in ensure_application_exists!': Xvfb not found on your system (Headless::Exception) from /Users/ct/.rvm/gems/ruby-2.1.1/gems/headless-1.0.2/lib/headless.rb:68:in

initialize 'from c_1319.rb: 9: in new' from c_1319.rb:9:in

'