Allow setting min/max SSL version for a connection on Ruby 2.5

[bdewater]

The functionality in the OpenSSL gem was introduced in ruby/openssl#142 and supported by Net::HTTP in Ruby 2.5: ruby/ruby@dcea919

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.

[bdewater]

Hi @drbrain 👋you're probably busy, but I was wondering if you might have a moment to look at this PR. Stripe's gem has started depending on net-http-persistent (stripe/stripe-ruby#698) and we would like to ensure client side that connections are not downgraded from TLS 1.2 for PCI-DSS reasons.

While we can run a fork, I think it would benefit the larger Ruby community to have this functionality merged. It might become a default setting in Stripe's gem for example.