fix erroneous loop and status check (also suggested by mgilfillan)

src/Client.php

@@ -289,9 +289,9 @@ class Client
             );
             $data = json_decode((string)$response->getBody(), true);
             sleep(1);
-        } while ($maxAttempts < 0 && $data['status'] == 'pending');
+        } while ($maxAttempts > 0 && $data['status'] == 'pending');
 
-        return (isset($data['status']) && $data['status'] == 'ready');
+        return (isset($data['status']) && $data['status'] == 'valid');
     }
 
     /**