Update obsidian textures

Before / after...

![old](https://cloud.githubusercontent.com/assets/9149204/5537466/7663abd0-8aa2-11e4-9487-de0fd7c0dba7.jpg)
![new](https://cloud.githubusercontent.com/assets/9149204/5537468/78902b7c-8aa2-11e4-9a17-ad1ce8d165fb.jpg)