Prevent betrad pixel trackers from messing with the layout

Sometimes this pixel tracker is inserted and it 404s:
```html
<img src="//l.betrad.com/ct/0_0_0_11429/gb/0/1/0/0/0/0/728/90/242/1685/0/pixel.gif?v=1159&amp;ttid=2&amp;d=www.theguardian.com&amp;r=0.34547341195866466">
```

It messes with the layout (see screenshots below).

![screen shot 2013-11-19 at 14 08 27](https://f.cloud.github.com/assets/85783/1572761/1b6f8388-5124-11e3-8ccc-7b38f4476918.PNG)


![screen shot 2013-11-19 at 13 45 54](https://f.cloud.github.com/assets/85783/1572752/fadfa65c-5123-11e3-9364-c28e66cffe05.PNG)

![ ](http://i.imgur.com/MdvIXKa.gif)