add z-index to secondary nav

The secondary nav is covered by body content in landing pages. Instead of fixing the issue on one specific page this is an attempt to fix all instances of a secondary nav. 
To verify, run a local verison of rizzo in landing pages. Resize your window to trigger a responsive menu on /bookings page and verify that the menu isn't covered Like :point_down: 
![screen shot 2014-10-06 at 9 19 36 am](https://cloud.githubusercontent.com/assets/2614324/4526950/df4b979c-4d63-11e4-8ffd-464f9fae894e.png)