[Testing] Fix required PHPUnit version

[kanafghan]

The required PHPUnit version in the docs should be updated to 4.2 (or later) since the atLeast() method which is used in some of the tests, e.g. Symfony\Component\HttpFoundation\Tests\Session\Storage\Handler\PdoSessionHandlerTest, is introduced in PHPUnit 4.2.0 (https://github.com/sebastianbergmann/phpunit/blob/4.2/src/Framework/TestCase.php#L1564).