Remove support for SSLv3 due to Poodle Attack

[guzmanbraso]

More info:

• https://poodle.io/
• https://community.rapid7.com/community/infosec/blog/2014/10/14/poodle-unleashed-understanding-the-ssl-30-vulnerability

SSLv3 support should be removed by default. If someone really need it it could use it by specifying a custom ssl_protocol argument.

[3flex]

Fixed by #480 (though SSLv3 is still on for mail by default).