fix search bar glitch in nav when using bourbon 4

From

![image](https://cloud.githubusercontent.com/assets/2095625/3066611/c36d75d8-e277-11e3-99b5-1b60505f51ff.png)

to

![image](https://cloud.githubusercontent.com/assets/2095625/3066612/c98e7c78-e277-11e3-8588-31af628b9ccd.png)