title: Frames issue using Watir-Webdriver and phantomjs
body: <p>I'm writing test using Watir-Webdriver and phantomjs and I've got an issue with iframes. 
  I'm trying to get an element with the following code:</p>
  
  <pre><code>browser.frame(:index =&gt; 0).div(:id, "wrapper").exists?
  </code></pre>
  
  <p>and when I'm using phantomjs, it raise an error:</p>
  
  <pre><code>NameError: uninitialized constant Watir::FramedDriver::UnknownFrameException
  </code></pre>
  
  <p>but when I'm using chrome driver, the code above works properly and returns "true" value.</p>
  
  <p>Any ideas how can I fix it? Or maybe there is any other way to get this element using phantomjs?
  Any help will be appreciated. Thanks!</p>