Need to serve site over HTTPS

[jbampton]

https://developer.mozilla.org/en-US/docs/Web/Progressive_web_apps

refs #85

On one of my GitHub Pages sites it looks like this:

[jbampton]

[matz]

Hmm, I tried to "Enforce HTTPS" in the configure page, but it says:

Unavailable for your site because your domain is not properly configured to support HTTPS (mruby.org)

I don't know how to fix this. Maybe I have to update the DNS setting.