Fix double border in navigation without children.

@jone 
**from**
![bildschirmfoto 2014-08-15 um 14 00 03](https://cloud.githubusercontent.com/assets/157533/3932895/bdd8cd1e-2473-11e4-8bbc-6f1af44c27f9.png)

**to**
![bildschirmfoto 2014-08-15 um 13 59 45](https://cloud.githubusercontent.com/assets/157533/3932896/c4bb539a-2473-11e4-83a7-1d7ee965f758.png)

( CSS + => sibling selector )