nginx-ssl.conf changes were partially incorrect, partial revert of 834c42bc

My own /etc/nginx/nginx.conf was messed up, so what I thought were Ubuntu 14.04 defaults weren't, and we lost the ssl_protocols and ssl_prefer_server_ciphers settings. This puts those back.

https://discourse.mailinabox.email/t/dev-master-version-reported-as-poodle-attack-vulnerable-by-ssllabs/898