title: MixItUp adds 'failed' class to div and does not filter items
body: <p>I'm trying to use <a href="https://github.com/patrickkunka/mixitup" rel="nofollow">MixItUp</a> to filter grid items. It says it should be pretty simple. Add the class <code>mix</code> and another class for the category of the item, and then add the buttons with the same category.</p>
  
  <pre><code>&lt;!-- The buttons --&gt;
  &lt;button class="filter" data-filter="social"&gt;Email&lt;/button&gt;
  &lt;button class="filter" data-filter="games"&gt;Games&lt;/button&gt;
  &lt;button class="filter" data-filter="merch"&gt;Merchandise&lt;/button&gt;
  
  &lt;!-- The grid --&gt;
  &lt;div class="portfolio-grid" id="MixItUpBCE8F4"&gt;
     &lt;div class="portfolio-item mix merch"&gt;&lt;/div&gt;
     &lt;div class="portfolio-item mix social"&gt;&lt;/div&gt;
     &lt;div class="portfolio-item mix games"&gt;&lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>I have initialised the plugin as such:</p>
  
  <pre><code>jQuery(document).ready(function($) {
      $(function(){
          $('.portfolio-grid').mixItUp();
      });
  });
  </code></pre>
  
  <p>And have added the described 'vital' CSS:</p>
  
  <pre><code>.portfolio-grid .mix{display: none;}
  </code></pre>
  
  <p>The plugin initialises, and the CSS applies.</p>
  
  <p>However, clicking any of the buttons simply causes the grid items to fade out. And no items show up on the page.</p>
  
  <h2><a href="https://jsfiddle.net/franhaselden/hq2j17yp/1/" rel="nofollow">JSFiddle</a></h2>
  
  <p>What have I done wrong? I've followed the docs to the letter.</p>
  
  <p>It seems to me that it's not matching any items. But I can't understand, because as you can see from the code above there are definitely matches as each item definitely has a class that matches one of the <code>data-filter</code>s?</p>