"GitHub Pages sites have been issued SSL certificates from Let's Encrypt, enabling HTTPS for your custom domain. This isn't officially supported yet and it's not possible for you to enable and enforce it on your sites at this time."
EDIT: found out the "official statement" here https://gist.github.com/coolaj86/e07d42f5961c68fc1fc8#gistco...
"GitHub Pages sites have been issued SSL certificates from Let's Encrypt, enabling HTTPS for your custom domain. This isn't officially supported yet and it's not possible for you to enable and enforce it on your sites at this time."