UI stackable menu does not stack the child "right menu"

The `right menu` inside `ui stackable menu` does not display correctly on mobile devices

``` html
<div class="ui stackable menu">
  <a class="item">Browse</a>
  <a class="item">Submit</a>
  <div class="right menu">
    <a class="item">Sign Up</a>
    <a class="item">Help</a>
  </div>
</div>
```

Example:
http://jsfiddle.net/7wpm7bka/
