rayTest should use btConcaveShape (not btTriangleMeshShape)

[jezek2]

Erwin, while I was working on the port I've found a small bug in original Bullet. In btCollisionWorld::rayTestSingle on line 314 (in 2.67), there is cast to btTriangleMeshShape instead of just btConcaveShape. I've found it when btStaticPlaneShape was encountered, as it throwed ClassCastException in Java.