Here is a simple method to fix Ruby 2.3.x OpenSSL connectivity errors (on Windows) when trying to install gems via rubygems. This method is the cleanest way to solve the problem and does not rely on you disabling any secure/https connectivity options in Ruby. Essentially, you are adding the missing CA certificate into the Ruby ssl_certs directory to complete the chain of trust.
Sample error messages:
*::Error::ConnectionFailed SSL_connect returned=1 errno=0 state=SSLv3 read server certificate B: certificate verify failed
or:
connect_nonblock': SSL_connect returned=1 errno=0 state=error: certificate verify failed (OpenSSL::SSL::SSLError)