YAML::XS not required any more by t/validation.t

It runs with YAML::Syck now too. I guess this is because of a fix in JSON::Validator/Swagger2::SchemaValidator. Can't reproduce old behaviour because metacpan seems to be broken and I don't know how to collect the backpan URLs.

So now the test should check for either YAML::XS or YAML::Syck.
