Something I miss on all these howto posts about Let's Encrypt is the revocation of the old certificate on renewal.

Say I have a cert that is valid until 2015-12-31, but I want to renew it now.

   ./letsencrypt-auto --agree-dev-preview --server \
      https://acme-v01.api.letsencrypt.org/directory certonly

will get a new certificate, but the old one, as far as I can see, is still valid and could be used for the domain until it expires.

letsencrypt-auto has a revoke function, but there is no easy way to tell it to revoke the last valid cert, or it should do it with a `renew` function.