fixed raycast bug, removed legacy voxel-engine 0.3.6 support

Now using simplified raycast approach instead of Game.prototype.raycast:

```
  // cheap raycast "lite":
  var stepDistance = 0.1
  var distanceChecked = 0
  var maxDistance = this.opts.distance || 10
  this.stepPosition.copy(this.game.cameraPosition())
  this.stepSegment.copy(this.game.cameraVector()).multiplyScalar(stepDistance)
  while (distanceChecked < maxDistance && !hit) {
    distanceChecked += stepDistance
    this.stepPosition.addSelf(this.stepSegment)
    if (this.game.voxels.voxelAtPosition(this.stepPosition)) {
      var hit = this.stepPosition
    }
  }
```

Also, using voxel size of 1 when calculating positions