[fixed] rebound's springSystem to use px/s
[fixed] rebound's springSystem to use px/s

Summary:
Rebound.JS [doesn't document](https://github.com/facebook/rebound-js/issues/29) what units it uses for `setVelocity()`, but it's ported over from Android.

The Android version [uses](https://github.com/facebook/rebound/blob/5017fc904cc9511384b69f79771386abf177bd37/rebound-android-playground/src/main/java/com/facebook/rebound/playground/examples/BallExample.java#L173) px/[second](https://developer.android.com/reference/android/view/VelocityTracker.html#computeCurrentVelocity%28int,%20float%29), so presumably the JS one does too.

Reviewers: O2 Material Motion, O3 Material JavaScript platform reviewers, #material_motion, featherless

Reviewed By: O2 Material Motion, #material_motion, featherless

Tags: #material_motion

Differential Revision: http://codereview.cc/D3164

material-motion/material-motion-js,material-motion/material-motion-js,material-motion/material-motion-js