Made SSL connections use the system certificate store by default.

- Set http.verify_mode to OpenSSL::SSL::VERIFY_PEER by default
- Use OpenSSL::X509::Store.new as certificate store unless another is specified

Adapted from Faraday.
See: https://github.com/lostisland/faraday/blob/b25adc0ca6b47eb5150494a735721579862990f0/lib/faraday/adapter/net_http.rb#L100