Dynamic verify mode based on Paypal environment

We have encountered **certificate verify failed** error when using paypal_adaptive with the Paypal Sandbox. However, the error doesn't occur when connecting to the production Paypal URL. The fix dynamically changes verify mode depending on the Paypal environment. When in Sandbox env, verify mode is OpenSSL::SSL::VERIFY_NONE. Else, OpenSSL::SSL::VERIFY_PEER, which is previously hardcoded value.