PayPal have disabled SSLv3, use TLS instead.

Since PayPal have disabled SSLv3, we have been seeing the occasional:

`OpenSSL::SSL::SSLError: SSL_connect returned=1 errno=0 state=SSLv3
read server certificate B: certificate verify failed`

by forcing TLS 1.2 as in other gateway providers, we are hoping this has fixed the issue.

We are open to better suggestions??