v0.8.8.  `step` function no longer binds `this` keyword.

Old style:

  tweenable.tween({
    from: {x: 0},
    to: {x: 10},
    step: function () {
      console.log(this.x);
    }
  });

New style:

  tweenable.tween({
    from: {x: 0},
    to: {x: 10},
    step: function (state) {
      console.log(state.x);
    }
  });