Lower portfolio next/prev links so they clear the header gradient

This prevents the top of the arrow links from being under the header gradient when the page isn't scrolled on tablet+ sizes.

From:
![screenshot 2014-03-28 15 34 01](https://cloud.githubusercontent.com/assets/90871/2554335/f1712ab8-b6af-11e3-98bc-281d316e8e3b.png)

To:
![screenshot 2014-03-28 15 33 40](https://cloud.githubusercontent.com/assets/90871/2554328/e17bee72-b6af-11e3-9543-ddb3a95e98bc.png)