PHPUnit_Framework_Assert::markTestSkipped() returns void, no need to return anything.

tests/Whoops/Handler/PrettyPageHandlerTest.php

@@ -290,7 +290,7 @@ class PrettyPageHandlerTest extends TestCase
             // Even though this test only uses ini_set and ini_get,
             // without xdebug active, those calls do not work.
             // In particular, ini_get after ini_setting returns false.
-            return $this->markTestSkipped('The xdebug extension is not loaded.');
+            $this->markTestSkipped('The xdebug extension is not loaded.');
         }
 
         $originalValue = ini_get('xdebug.file_link_format');