Support for the new RSpec3 expectation protocol.

RSpec3 keeps yelling that:

    Mongoid::Matchers::Validations::ValidateUniquenessOfMatcher implements a legacy RSpec matcher
    protocol. For the current protocol you should expose the failure messages
    via the `failure_message` and `failure_message_when_negated` methods.

So I added the `failure_message` and `failure_message_when_negated` method aliases to the needed classes.