More subtle scroll shadow

I'm worried that the current scroll shadow is a bit too strong, and might obstruct the text. This makes it more subtle.

Before:

![screen shot 2014-09-11 at 10 30 42](https://cloud.githubusercontent.com/assets/211284/4231332/ee88e04e-398f-11e4-8626-38ded53cc437.png)

After:

![screen shot 2014-09-11 at 10 33 41](https://cloud.githubusercontent.com/assets/211284/4231335/f4b70680-398f-11e4-90c8-2dbe251c5aaa.png)