Removes rebound-js

Since July 25 there's a specific repo for [rebound-js](https://github.com/facebook/rebound-js). As it is not being used here (i supposed that it would be used in an example using WebView, but there are any examples using - even if it was, we wouldn't need to keep it here as well) , i think that there's no reason to continue mantaining it here.

In this PR i also updates the Readme so that the rebound-js repo is referenced. 

What do you think guys?