tweaked .floatingheader background for non-webkit browsers

Replaced the .floatingheader background rules with a [Bourbon linear-gradient mix-in](http://bourbon.io/docs/#linear-gradient)

before/after image:

![floatingheader-nonwebkit-fix](https://f.cloud.github.com/assets/263119/1337108/750ba3a4-35d1-11e3-936f-76994d585a32.png)