bump required PHPUnit version

Author: xabbuh

book/testing.rst

@@ -17,7 +17,7 @@ it has its own excellent `documentation`_.
 
 .. note::
 
-    Symfony works with PHPUnit 3.5.11 or later, though version 3.6.4 is
+    Symfony works with PHPUnit 3.5.11 or later, though version 4.2 is
     needed to test the Symfony core code itself.
 
 Each test - whether it's a unit test or a functional test - is a PHP class