Make GitHub ribbon clickable in small viewports

Builds on top of #11 (please merge it first), so just look at 964b6f3fa92c60980e8ad2fec210698a11c69067 - the second commit.

At around 1240px wide the GitHub fork me ribbon div starts to slide under the LibSass logo and gradually becomes unclickable. I noticed this because I have my browser at half the screen width most of the time, and kept being taken back to [libsass.com](http://libsass.com) while clicking the ribbon.

The two obvious solutions to me are to either move the ribbon below the LibSass logo, or to add `z-index:1` to the style attribute of the image. I chose the first option, but I can change it to the `z-index` option instead if preferred.