(#19151) Reject SSLv2 and MD5 SSL handshakes

Without this patch, SSL connections on older versions of Ruby will
negotiate down to insecure modes of operation, specifically SSLv2.  This
is a problem because SSLv2 needs to be rejected outright to meet
security policies.

This patch addresses the problem by changing the behavior of the
OpenSSL::SSL::SSLContext class.  With this patch applied, all SSLContext
objects will be initialized with a default cipher rule set of
ALL:!aNULL:!eNULL:!ADH:!EXPORT:!LOW:!SSLv2:!MD5:+TLSv1:+SSLv3 and with
SSLv2 explicitly disabled using the options flags.

This change is a monkey patch to MRI Core and will affect all SSL socket
clients and servers.  The options and cipher list may still be
explicitly set by passing an options hash with the :options and :ciphers
keys to the SSLContext@set_params method.