initial rake db:migrate was failing if hstore was already set

db/migrate/20120921162512_add_meta_data_to_forum_threads.rb

@@ -1,6 +1,6 @@
 class AddMetaDataToForumThreads < ActiveRecord::Migration
   def change
-    execute "CREATE EXTENSION hstore" 
+    execute "CREATE EXTENSION IF NOT EXITS hstore" 
     add_column :forum_threads, :meta_data, :hstore
   end
 end