Fix navigation menu margin for last child

Before:

![image](https://cloud.githubusercontent.com/assets/126538/2813177/bf91d542-ce7e-11e3-9288-08c1e18ceb12.png)

After:

![image](https://cloud.githubusercontent.com/assets/126538/2813180/e579dc78-ce7e-11e3-8bf1-077a18d3da6f.png)