Change the visibility of WebTestCase::setUp to keep the same as in PHPUnit

see https://github.com/sebastianbergmann/phpunit/blob/ea293f782b3e001c9b7eeb5bdc2d545e4c091fdb/src/Framework/TestCase.php#L1834

src/Silex/WebTestCase.php

@@ -29,7 +29,7 @@ abstract class WebTestCase extends \PHPUnit_Framework_TestCase
      * Note: Child classes that define a setUp method must call
      * parent::setUp().
      */
-    public function setUp()
+    protected function setUp()
     {
         $this->app = $this->createApplication();
     }