Fix bug with 'more...' button on related keywords

Fixes this 

![screen shot 2014-07-28 at 11 28 59](https://cloud.githubusercontent.com/assets/1001642/3719347/0b80edc0-1642-11e4-86dc-87b9cad693f2.png)

bug in Firefox
@kaelig I was wondering whether this `content__keywords--more-tags` is correctly named to begin with.

Example HTML to which this is applied:

```html
<div class="content__keywords" data-link-name="article keywords">
  <ul class="inline-list">
    <li class="inline-list__item">Tags:</li>
    <li class="inline-list__item ">
      <a class=" " href="http://www.theguardian.com/politics/general-election-2015" data-link-name="keyword: politics/general-election-2015" itemprop="keywords">General election 2015</a>,
    </li>
    <li class="inline-list__item js-more-tags">
      <button class="u-button-reset u-fauxlink content__keywords--more-tags js-more-tags__link" data-link-name="more-tags">more…</button>
    </li>
  </ul>
</div>
```