Stop nav list items wrapping across columns

![screen shot 2014-10-20 at 15 13 51](https://cloud.githubusercontent.com/assets/867233/4702500/6e2264d2-5863-11e4-8210-9dd99f505bef.png)

becomes

![screen shot 2014-10-20 at 15 14 02](https://cloud.githubusercontent.com/assets/867233/4702513/7d366fcc-5863-11e4-9a5a-c11f961c6e6f.png)