Memoize OrderBalance instance

There's no need to create an instance of such class for every call to
one of its methods. The balance is computed each time anyway, so it'll
always be up-to-date.

app/models/concerns/balance.rb

@@ -9,7 +9,7 @@ module Balance
 
   # Branches by the OrderBalance abstraction
   def outstanding_balance
-    OrderBalance.new(self)
+    @order_balance ||= OrderBalance.new(self)
   end
 
   # Returns the order balance by considering the total as money owed to the order distributor aka.