Fix DIRECTORY_SEPARATOR bug occurring on Windows

Fixes #3:
The sheetN.xml files location is dynamically retrieved when parsing
[Content_Types].xml. In this file, the location is like "/xl/worksheets/sheet1.xml".

Because the zip stream wrapper expects the zip entry name to be like
"xl/worksheets/sheet1/xml" (without the first "/"), this first "/"
needs to be trimmed. It was trimmed using:
  ltrim($this->dataXmlFilePath, DIRECTORY_SEPARATOR);
which obvisously does not work on Windows platform where DIRECTORY_SEPARATOR is "\".
Replacing DIRECTORY_SEPARATOR by '/' solves this issue.

src/Spout/Reader/Internal/XLSX/Worksheet.php

@@ -31,7 +31,7 @@ class Worksheet
      */
     public function getDataXmlFilePath()
     {
-        return ltrim($this->dataXmlFilePath, DIRECTORY_SEPARATOR);
+        return ltrim($this->dataXmlFilePath, '/');
     }
 
     /**