column-coordinates greater than Z are not working

This is a bug report!

Column-coordinates greater than Z are not working

```php
<?php
require __DIR__ . '/vendor/autoload.php';
// Create new Spreadsheet object
$spreadsheet = new \PhpOffice\PhpSpreadsheet\Spreadsheet();
...
$activeSheet->mergeCells("Z1:AA1");
```

### Which versions of PhpSpreadsheet and PHP are affected?
1.3.0
