Demo.html isn't working anymore, due to mixing HTTP and HTTPS requests

[jules43]

Line 25 of demo.html triggers a mixed content exception due to 'http://maps.stamen.com/js/tile.stamen.js' being loaded with http.

From this post:
https://stackoverflow.com/questions/48874337/how-to-use-leaflet-js-plugin-with-stamen-maps

The secure version is here:

<script src="https://stamen-maps.a.ssl.fastly.net/js/tile.stamen.js"></script>