Default the built-in Jenkins Update Center URL to https://updates.jenkins.io

Now that we're using JDK8, we can rely on our Let's Encrypt-based certificates
on *.jenkins.io

Live from Jenkins World!

Signed-off-by: M. Allan <max.allan@surevine.com>
Signed-off-by: R. Tyler Croy <tyler@monkeypox.org>