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

libraries/phpspreadsheet/vendor/composer/autoload_psr4.php

@@ -0,0 +1,13 @@
+<?php
+
+// autoload_psr4.php @generated by Composer
+
+$vendorDir = dirname(dirname(__FILE__));
+$baseDir = dirname($vendorDir);
+
+return array(
+    'Psr\\SimpleCache\\' => array($vendorDir . '/psr/simple-cache/src'),
+    'PhpOffice\\PhpSpreadsheet\\' => array($vendorDir . '/phpoffice/phpspreadsheet/src/PhpSpreadsheet'),
+    'Matrix\\' => array($vendorDir . '/markbaker/matrix/classes/src'),
+    'Complex\\' => array($vendorDir . '/markbaker/complex/classes/src'),
+);