Installed this way:
git clone https://github.com/letsencrypt/letsencrypt /opt/letsencrypt
mkdir /var/www/acme/
chown www-data:www-data /var/www/acme
/opt/letsencrypt/letsencrypt-auto certonly -a webroot --webroot-path=/var/www/acme/ -d yourdomain.com -d www.yourdomain.com