insecure strapdown.js URL prevents website rendering on Chrome

The https://green-haskell.github.io website does not render with Google Chrome:

> mixed Content: The page at 'https://green-haskell.github.io/' was loaded over HTTPS, but requested an insecure script 'http://strapdownjs.com/v/0.2/strapdown.js'. This request has been blocked; the content must be served over HTTPS.