update for PHP 7.1

this fixes the "A non-numeric value encountered" warning introduced in
PHP 7.1.

This fixes the test

    ean13_generator_throws_exception_if_invalid_chars_are_used

Author: pilif

src/BarcodeGenerator.php

@@ -1502,7 +1502,7 @@ protected function barcode_eanupc($code, $len = 13)
         }
         $sum_b = 0;
         for ($i = 0; $i < $data_len; $i += 2) {
-            $sum_b += ($code[$i]);
+            $sum_b += intval(($code[$i]));
         }
         if ($len < 13) {
             $sum_b *= 3;