Fix rake db:init.

This is kind of a hack and I'm not sure why it's undefined in the Rake task
because it works when running the app, and #{field}_changed? is defined by
Mongoid as part of its dirty tracking implementation.

models/comment_thread.rb

@@ -57,7 +57,7 @@ class CommentThread < Content
   validate :tag_names_unique
 
   before_create :set_last_activity_at
-  before_update :set_last_activity_at, :unless => closed_changed?
+  before_update :set_last_activity_at, :unless => lambda { closed_changed? }
 
   before_destroy :destroy_subscriptions