For Nginx 1.9.5 and later, change
listen 443 ssl;
listen 443 ssl http2;
and you are all set.
Note that all implementations of HTTP/2 require HTTPS.
Ubuntu 16.04 LTS already comes with Nginx 1.10.
To automatically renew certificates from letsencrypt, just run
sudo /path/to/letsencrypt-auto renew 1>/dev/null 2>&1 && sudo nginx -s reload
periodically if you have initiated the configurations correctly. 🙂
By the way, enabling HTTP/2 does not seem to bring any good to my websites. However it does make me feel good. 😉