Fixed PHP7 Depecated Warnings

include/Excel/Workbook.php

@@ -171,10 +171,10 @@ class Spreadsheet_Excel_Writer_Workbook extends Spreadsheet_Excel_Writer_BIFFwri
     * @param string filename for storing the workbook. "-" for writing to stdout.
     * @access public
     */
-    function Spreadsheet_Excel_Writer_Workbook($filename)
+    function __construct($filename)
     {
         // It needs to call its parent's constructor explicitly
-        $this->Spreadsheet_Excel_Writer_BIFFwriter();
+        parent::__construct();
 
         $this->_filename         = $filename;
         $this->_parser           = new Spreadsheet_Excel_Writer_Parser($this->_byte_order, $this->_BIFF_version);