Try loading required JS libraries in the following order, if anyone still having issues in showing the DataTable export buttons.
<script type="text/javascript" src="https://cdn.datatables.net/buttons/1.3.1/js/dataTables.buttons.min.js"></script>
<script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/jszip/3.1.3/jszip.min.js"></script>
<script type="text/javascript" src="https://cdn.datatables.net/buttons/1.3.1/js/buttons.html5.min.js"></script>
Cheers!