$ sudo letsencrypt certonly --manual -d <domain>
You'll need to deploy a verification file to your current website.
http://<website>/.well-known/acme-challenge/<filename-given-by-command>
The contents of the file are provided during the interactive prompts.
$ sudo heroku certs:add --app <app> /etc/letsencrypt/live/<domain>/cert.pem /etc/letsencrypt/live/<domain>/privkey.pem
$ sudo heroku certs:update --app <app> /etc/letsencrypt/live/<domain>/cert.pem /etc/letsencrypt/live/<domain>/privkey.pem