Let's Encrypt uses DNS to verify domain ownership, so DNS records must
be set up before the paybook is run the first time.

Make a clean distinction between Debian 7 and Debian 8.  Anticipate the
next Ubuntu LTS release (Xenial) that is planned for support.

I ran a check on http://mail-tester.com and found out I was missing a header to be set for DMARC. SInce I missed it, maybe someone else missed it as well, so maybe it could go into the README.

Not awesome. We need a better alternative, ideally one with a better security stance.

Reflects the move to the `sovereign` GitHub organization.

Addresses #361.

Closes #343.