Support PHPUnit 6

tests/Spout/Writer/Common/Entity/SheetTest.php

@@ -5,11 +5,12 @@ namespace Box\Spout\Writer\Common\Entity;
 use Box\Spout\Common\Helper\StringHelper;
 use Box\Spout\Writer\Common\Manager\SheetManager;
 use Box\Spout\Writer\Exception\InvalidSheetNameException;
+use PHPUnit\Framework\TestCase;
 
 /**
  * Class SheetTest
  */
-class SheetTest extends \PHPUnit_Framework_TestCase
+class SheetTest extends TestCase
 {
     /** @var SheetManager */
     private $sheetManager;