The reason why you might get certificate errors in Ruby 2.0 when talking HTTPS is because there isn't a default certificate bundle that OpenSSL (which was used when building Ruby) trusts.
Update: this problem is solved in edge versions of rbenv and RVM.
$ ruby -rnet/https -e "Net::HTTP.get URI('https://github.com')" net/http.rb:917:in `connect': SSL_connect returned=1 errno=0 state=SSLv3 read server certificate B: certificate verify failed (OpenSSL::SSL::SSLError)
You can work around the issue by installing a certificate bundle that you trust. I trust Mozilla and curl.