Fix nav splits if there are only 6 items.

#### What's this PR do?
Fixes an issue where if there were only 6 items in the nav, the last cell would only occupy 1/2 of it's space.
#### Where should the reviewer start?
css.
#### How should this be manually tested?
You can see it locally at http://localhost:8000/psf-landing/
#### Any background context you want to provide?
I'm unsure on what browser support looks like for this site. I think not selectors are only back to ie8 or so. I don't think this is possible as a CSS only change.
#### What are the relevant tickets?
Fixes #491 
#### Screenshots (if appropriate)
Links to prod version in #491 
Updated:
![screen shot 2014-12-14 at 3 42 19 pm](https://cloud.githubusercontent.com/assets/3853/5429637/dfe79bc2-83a7-11e4-9a30-d163d00cd634.png)

#### What gif best describes this PR or how it makes you feel?
![](http://s.mlkshk-cdn.com/r/11WCB.jpg)