update nginx configs to improve SSL/TLS security

* prefer ciphers that use perfect forward secrecy
* disable SSLv3. This will break SSL for ie6+xp (ore older) users,
  that's < 0.05% of our users. It's required thanks to the new POODLE
  vulnerability

  http://googleonlinesecurity.blogspot.com/2014/10/this-poodle-bites-exploiting-ssl-30.html
  http://blog.cloudflare.com/sslv3-support-disabled-by-default-due-to-vulnerability/