https://certificate.revocationcheck.com/helloworld.letsencrypt.org shows helloworld.letsencrypt.org is correctly stapling.
That machine is running nginx >= 1.8, and has this configuration:
# Site-Specific HTTPS configuration
ssl_certificate /etc/letsencrypt/live/helloworld.letsencrypt.org/fullchain.pem;
ssl_certificate_key /etc/letsencrypt/live/helloworld.letsencrypt.org/privkey.pem;
ssl_trusted_certificate /etc/letsencrypt/live/helloworld.letsencrypt.org/fullchain.pem;
ssl_stapling on;
ssl_stapling_verify on;
Hope that helps!