Add support for HTTPS on builds.wpt.fyi

[jugglinmike]

This is generally a good practice, but the lack of support is currently blocking a security enhancement for https://wpt.fyi--the introduction of the HSTS header (see gh-517).

Expected implementation: Let's Encrypt via CertBot

[foolip]

This is blocking https://github.com/web-platform-tests/wpt.fyi/issues/58.

An alternative is to use another domain name. I have no preference :)