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

libraries/phpspreadsheet/vendor/psr/simple-cache/src/CacheException.php

@@ -0,0 +1,10 @@
+<?php
+
+namespace Psr\SimpleCache;
+
+/**
+ * Interface used for all types of exceptions thrown by the implementing library.
+ */
+interface CacheException
+{
+}