change invoke to construct in stubs

[mnshankar]

Hi Joe,
Thanks for this great package! There is one small issue though.. the files that are generated under webhookjobs get created as follows:

<?php
namespace App\WebhookJobs;
class NetAuthorizeCustomerSubscriptionExpiring
{
    public function __invoke($eventPayload) {
        //
    }
}

They all should have

public function  __construct($eventPayload){
}

Thanks again for your work on this!
Shankar.