Hi, did you follow the instructions at https://certbot.eff.org/ or somewhere else? I’m going to guess somewhere else, as letsencrypt-auto has been replaced by certbot-auto for some time now.
If you follow the instructions at that site, you’ll get information on using an official PPA to get the certbot client. Alternately, you can still use certbot-autoby using the instructions here. I tested these on an Ubuntu 14.04 setup and they work great.