Support setting SSLContext#min_version and #max_version

[bdewater]

The functionality in the OpenSSL gem was introduced in ruby/openssl#142 and supported by Net::HTTP in Ruby 2.5: https://github.com/ruby/ruby/commit/dcea9198a9d80bdf4eeacd9d9e9d883850a4a8d2

An example why this might be useful; for payment data the PCI DSS mandates that TLS 1.1 or newer is used after June 30. Using ssl_version would disallow the client negotiating TLS 1.2 (or 1.3 in the near future) if both sides support it, min_version doesn't have this problem.

[gshutler]

Another use case for this is the reverse.

The official Ruby images include default OpenSSL configuration restricting the min version to TLSv1.2. We talk to some old servers out of our control that cannot satisfy this requirement and would want to permit this at the code level. Currently we have to change the default at the OS level rather than allowing it in a more targetted fashion.