Update example server to use TLS 1.2

[igrigorik]

Existing implementation (example/server.rb) defaults to weak ciphersuite that triggers INADEQUATE_SECURITY in modern browsers. That said, I'm having a hard time navigating the Ruby OpenSSL implementation to figure out how to set all the right parameters: http://httpwg.org/specs/rfc7540.html#rfc.section.9.2.1

/cc @tenderlove @tarcieri perhaps you guys can help? :)

[tarcieri]

I had someone just today complaining about the OpenSSL extension's default ciphersuite list as well:

httprb/http#334

I've tried to make a few improvements in the past, e.g:

ruby/openssl#44

...but it's in definite need of a tune-up, as are the default TLS parameters.

/cc @zzak @nahi @emboss