No TLS on jenkins-x.com, tisk tisk.
Given that Jenkins is pretty popular, you'd think that they'd be able to sort something out with GitHub to get bumped up the list for something along these lines.
There's always the Cloudfare option, but I've never felt that this was an ideal solution when HTTPS should be extremely straightforward for GitHub to set up on their pages.
Looking at the dns records, it looks like they didn't do this, and instead set up an A record.
I realized Github probably documents this, and found: https://help.github.com/articles/using-a-custom-domain-with-...
My suggestion would likely work for a www subdomain, but not for the apex domain.