Issue #2826 - PayPal Payment Data not stored

PP Payment Details aren't being stored on posts meta table, because of a probably misstyped
variable on class-wc-gateway-paypal.php, in the function successful_request (Lines 658 to 666).

Replaced $order_id (non existent variable) by $order->id to correct the issue.
Merge pull request #2827 from KveRa/master

Issue #2826 - PayPal Payment Data not Stored