Fix warnings with latest PHPExcel develop version

Fixes this error:

[RuntimeException]
Error Output: PHP Fatal error:  Access level to Maatwebsite\Excel\Readers\Html::$_inputEncoding must be protected (as in class PHPExcel_Reader_HTML) or weaker in /secret-project/vendor/maatwebsite/excel/src/Maatwebsite/Excel/Readers/HtmlReader.php on line 33

when using latest git version of PHPOffice/PHPExcel