adds migration to remove AaCwT columns

it migrates existing comments to ancestry

db/migrate/20150902120006_remove_parent_id_from_comments.rb

@@ -0,0 +1,9 @@
+class RemoveParentIdFromComments < ActiveRecord::Migration
+  def change
+    Comment.build_ancestry_from_parent_ids! rescue nil
+
+    remove_column :comments, :parent_id, :integer
+    remove_column :comments, :lft, :integer
+    remove_column :comments, :rgt, :integer
+  end
+end