Use new created property on invoice

The date property was removed from the invoice object in https://stripe.com/docs/upgrades#2019-03-14

src/Invoice.php

@@ -45,7 +45,7 @@ public function __construct($owner, StripeInvoice $invoice)
      */
     public function date($timezone = null)
     {
-        $carbon = Carbon::createFromTimestampUTC($this->invoice->date);
+        $carbon = Carbon::createFromTimestampUTC($this->invoice->created ?? $this->invoice->date);
 
         return $timezone ? $carbon->setTimezone($timezone) : $carbon;
     }