Use TLS 1.2 to default version of OpenSSL
OpenSSL on trunk still use SSL version 3 with default option. but SSLv3 have some vulnerability.
I propose to use TLS 1.2 with default on OpenSSL library.
see original proposal: https://github.com/ruby/ruby/pull/873
In other side, HTTP/2 must be required TLS 1.2 protocol. We should change it before http client
ctx.ssl_version = :TLSv1_2 every their code.