Add setPosition by SetTransform to fix NoMethodError in Joybox

To fix this code raises NoMethodError.
https://github.com/rubymotion/Joybox/blob/master/lib/joybox/box2d/body.rb#L83

I refer this topic. if I have any problem, please tell me.
http://www.cocos2d-iphone.org/forum/topic/1924