Allow access to Serializer's visitors getter

[adrienbrault]

Since 1.0.x, I can no longer do

<?php

$container->get('jms_serializer.xml_serialization_visitor')->setDefaultRootName('a_root_name');

because jms_serializer.xml_serialization_visitor is now private.

With this PR I could just do

<?php

$container->get('serializer')->getSerializationVisitor('xml')->setDefaultRootName('a_root_name');

[adrienbrault]

Build fixed

[schmittjoh]

Thanks!