fix hover on primary nav

![screenshot_53](https://cloud.githubusercontent.com/assets/1038697/4010662/0a728e3c-29f9-11e4-9bb4-e68c124c03ef.png) => ![screenshot_54](https://cloud.githubusercontent.com/assets/1038697/4010666/11ab11f6-29f9-11e4-9876-2405b95cf8d4.png)