There are several plugins that can do this.
Whith my plugin Stamp, you can add a timestamp to the filename or path in the following styles:
<script src="/assets/build/js/1399647655/scripts.js"></script>
<script src="/assets/build/js/scripts.1399647655.js"></script>
<script src="/assets/build/js/scripts.js?ts=1399647655"></script>
The last one, which is the one you're looking for is made with:
<script src="{{ craft.stamp.er('/assets/build/js/scripts.js', 'query') }}"></script>
You can also get only the timestamp, and use it however you want, like this:
{% set timestamp = craft.stamp.er('/assets/build/js/scripts.js', 'tsonly') %}