title: Webdriver.io: isExisting().then() is not a function
body: <p>I would like to check if there is a logout element. If it is existing, I want to do the logout by clicking this element:</p>
  
  <pre><code>browser.isExisting('.logout').then(function() {
      browser.click('.logout');
  });
  </code></pre>
  
  <p>But this gives me an <code>Uncaught TypeError: browser.isExisting(...).then is not a function</code>-error.</p>