Fix tax adjustments rounding error

Related PR: https://github.secureserver.net/PC/nemo/pull/103
This method is duplicated order_updater.rb. Can we make it DRY? I'm not sure what's a good place for this. How about making a util class with this method accessed as `Spree::Utils.round_money`? It's not the best, but at least will avoid repetition for now.