fix issue, related to hard-coded rollaxis index, should use m_indexForwardAxis

Thanks Joerg, luding.org

src/BulletDynamics/Vehicle/btRaycastVehicle.cpp

@@ -691,7 +691,7 @@ void	btRaycastVehicle::updateFriction(btScalar	timeStep)
 					
 					btVector3 sideImp = m_axle[wheel] * m_sideImpulse[wheel];
 
-					rel_pos[2] *= wheelInfo.m_rollInfluence;
+					rel_pos[m_indexForwardAxis] *= wheelInfo.m_rollInfluence;
 					m_chassisBody->applyImpulse(sideImp,rel_pos);
 
 					//apply friction impulse on the ground