Use ValidatorBuilder for ValidatorServiceProvider

This makes it easier to override different stuff for the validation also use different validator api's.

The failing test is from the Locale stuff.
even in this case, such BC break is bad (people registering custom initializers by overwriting this array will see that things don't work anymore without having any notice that Silex changed the way to register them.

Given that supporting this array is a no-brainer even when using the ValdiatorBuilder, I think we could avoid dropping it.