1 - Download the certificate with:
wget -O config/rds-combined-ca-bundle.pem https://s3.amazonaws.com/rds-downloads/rds-combined-ca-bundle.pem
2 - Add the certificate file to the repository and redeploy to Heroku.
3 - Update the DATABASE_URL env var by appending ?sslmode=verify-full&sslrootcert=config/rds-combined-ca-bundle.pem
References: