Fixed fatal error in paypal gateway

This line should contain use $response instead of the not existing $result.

The error only occurs when debugging is enabled and the communication with Paypal API failed for some reason. But in this case, it results in a fatal php error: Call to a member function get_error_message() on a non-object
Merge pull request #2151 from welovewordpress/patch-1

Fixed fatal error in paypal gateway