Merge branch '3.4' into 4.1

* 3.4:
  [travis][appveyor] use symfony/flex to accelerate builds
  Add missing stderr redirection
  clean up unused code
  [Filesystem] Add test to prevent regression when using array|resource with dumpFile
  [Security] Call AccessListener after LogoutListener

Author: nicolas-grekas

Tests/CoverageListenerTest.php

@@ -12,7 +12,7 @@ public function test()
             $this->markTestSkipped('This test cannot be run on Windows.');
         }
 
-        exec('type phpdbg', $output, $returnCode);
+        exec('type phpdbg 2> /dev/null', $output, $returnCode);
 
         if (\PHP_VERSION_ID >= 70000 && 0 === $returnCode) {
             $php = 'phpdbg -qrr';