Upgrade to hibernate-validator 5.1.1

Since we're going from 5.0.x to 5.1.x, here's the 5.1.0 announcement:
http://in.relation.to/Bloggers/DontWaitUpgradeHibernateValidator510FinalIsOut

Note that Grails can't use version 5.1.2 (5.1.2 is currently the latest) because that version doesn't work on Java 6, see https://hibernate.atlassian.net/browse/HV-924

Using dependency management, I forced my (large) application to use hibernate-validator 5.1.1 and it worked (all unit, integration, and other tests passed) so I believe this change to be fine.