fixed gameplay on faster machines

So, the jam is over and I have had mixed feedback on the game. One thing that came up a lot was that for some players the speed of the game was insanely fast.  

This is a quick fix that caps the FPS for the game to 60. The physics update logic is actually performed every frame, so higher framerates meant faster game speed.

I will be looking for a better solution, but for now this will ensure everyone will get about the same experience from the game.