Use 3.4's ssl.create_default_context when available

Related #472 
``ssl_wrap_socket`` already silently ignores parameters on Pythons that don't support different feature(e.g., SNI). I'm 100% in favor of doing the same here because we use this in ``ssl_wrap_socket``. If a user, however, creates a SSLContext object, then I have no issue raising the exception there.