Use 3.4's ssl.create_default_context when available

Related #472 
FWIW I don't feel strongly about using that if a use can pass ``SSLContext``s in on their own. I was just thinking long term it'd be nice to be able to not have any of this code (although that's much longer term since ``create_default_context`` is only in 3.4+ and 2.7.9+).