Dont let cavegen carve caves high above surface.

Cavegen v6 is still making caves up in the sky...
![screenshot_3261052666](https://cloud.githubusercontent.com/assets/2177790/3872036/d26b9d56-210d-11e4-82b6-6322c3a2e8a3.png)
Here i replaced cave air with water to make them visible.