create mongoDB indexes for locomotive models without any errors

app/models/locomotive/extensions/page/tree.rb

@@ -18,7 +18,7 @@ module Locomotive
 
           ## indexes ##
           index :position
-          index [:depth.asc, :position.asc]
+          index [[:depth, Mongo::ASCENDING], [:position, Mongo::ASCENDING]]
         end
 
         module ClassMethods

lib/tasks/locomotive.rake

@@ -60,4 +60,5 @@ namespace :locomotive do
     end
 
   end
+
 end