Validate both the model and its translations

This way we guarantee there will be at least one translation for a model
and we keep compatibility with the rest of the application, which
ideally isn't aware of globalize.

app/models/admin_notification.rb

@@ -5,11 +5,8 @@ class AdminNotification < ActiveRecord::Base
   translates :body,  touch: true
   include Globalizable
 
-  translation_class.instance_eval do
-    validates :title, presence: true
-    validates :body, presence: true
-  end
-
+  validates_translation :title, presence: true
+  validates_translation :body, presence: true
   validates :segment_recipient, presence: true
   validate :validate_segment_recipient