There are plans to enable HTTPS on the main site, but it will happen when people have time.
https://github.com/cuberite/cuberite.git.../issues/50
Edit: Bearbin had a solution to the problem (proxy hosted on the build server). It should be as simple as updating the IP address of the main site.
Second edit: This is how it would work: Change the proxy on the build server to point at http://cuberite.github.io, remove the CNAME file in the cuberite.github.io repo, change every mention of http://cuberite.org to https://cuberite.org in the cuberite.github.io repo, update the IP of the main site to the build server's.
https://github.com/cuberite/cuberite.git.../issues/50
Edit: Bearbin had a solution to the problem (proxy hosted on the build server). It should be as simple as updating the IP address of the main site.
Second edit: This is how it would work: Change the proxy on the build server to point at http://cuberite.github.io, remove the CNAME file in the cuberite.github.io repo, change every mention of http://cuberite.org to https://cuberite.org in the cuberite.github.io repo, update the IP of the main site to the build server's.