uniqueness: don't use a private Rails API

`ActiveRecord::Validations#_validations` is a private API.

Fortunately, Rails provides an API that does exactly what's needed here,
and it's been in Rails since at least 3.0.0, so let's use that instead.

lib/shoulda/matchers/active_record/validate_uniqueness_of_matcher.rb

@@ -396,7 +396,7 @@ def description_for_case_sensitive_qualifier
         end
 
         def validations
-          model._validators[@attribute].select do |validator|
+          model.validators_on(@attribute).select do |validator|
             validator.is_a?(::ActiveRecord::Validations::UniquenessValidator)
           end
         end