To make the cert auto renew in Ubuntu 14.04 LTS, I did this:
cd /etc/cron.daily
nano letsencrypt-renew
and added to the file this renew command, taken from the certbot docs:
#!/bin/bash
/var/letsencrypt/certbot-auto renew --quiet --no-self-upgrade
I then had to change permissions on the file so it could run:
chmod 755 letsencrypt-renew
After that I was able to run the bash script manually and it seemed to work! It’s only daily, the docs recommend twice a day, but once a day seems like enough renewal opportunities to me, since the certs expire every 90 days.