Resolved gh-444 by adding the new phpspreadsheet library, and removing the old PHPExcel library.

libraries/phpspreadsheet/vendor/autoload.php

@@ -0,0 +1,7 @@
+<?php
+
+// autoload.php @generated by Composer
+
+require_once __DIR__ . '/composer/autoload_real.php';
+
+return ComposerAutoloaderInit6afe6fcd54e00add00a5fe0e5fb044f9::getLoader();