title: Cucumber tests fails to start with error "Display socket is taken but lock file is missing.."
body: <p>After running cucumber </p>
  
  <blockquote>
    <p>bundle exec cucumber features/emails.feature:20</p>
  </blockquote>
  
  <p>I am getting error</p>
  
  <blockquote>
    <p>Display socket is taken but lock file is missing - check the Headless
    troubleshooting guide (Headless::Exception)
    /Users/me/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/headless-2.2.0/lib/headless.rb:195:in
    <code>ensure_xvfb_is_running' 
    /Users/me/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/headless-2.2.0/lib/headless.rb:184:in
    </code>launch_xvfb'
    /Users/me/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/headless-2.2.0/lib/headless.rb:172:in
    `block in pick_available_display'</p>
  </blockquote>
  
  <p>I am running MacOS 10.11.5, xquartz installed, test started first time and I cant make it to start again. Any suggestions?</p>