Replaced the gem mine artwork

Railmine does not work well together with the resource layer. This keeps it as a decorative element, but replaces the gem mine with new 1x1 artwork created by @MicroJOo.

![gem-mine](https://cloud.githubusercontent.com/assets/756669/4783262/f572091e-5d1e-11e4-9c1d-3838609c6354.png)

Sounds good. Updated.