Retrieve XLSX sheets in order of appearance (#220)

Instead of relying on the ID, sheets should be retrieved in the order they appear in the file.
Workbook.xml describes the correct order.
This allows the reader to read data in the correct order when sheets have been manually moved after creation.

tests/Spout/Reader/XLSX/ReaderTest.php

@@ -23,7 +23,7 @@ class ReaderTest extends \PHPUnit_Framework_TestCase
     {
         return [
             ['/path/to/fake/file.xlsx'],
-            ['file_with_no_sheets_in_content_types.xlsx'],
+            ['file_with_no_sheets_in_workbook_xml.xlsx'],
             ['file_with_sheet_xml_not_matching_content_types.xlsx'],
             ['file_corrupted.xlsx'],
         ];