Order budgets by created_at instead of by id

This is a preventive change which will be useful once the rake to
migrate from `spending_proposals` to `budget_investments` is complete

As after running that migration, old `spending_proposal` budgets will
have a newer `id` than the existing budgets. And therefore the last
budget will be one of those migrated from the old `spending_proposal`
model

By ordering by `created_at` and probably updating the `created_at`
attribute in the rake that migrates `spending_proposals` to
`budget_investments`, we will have a coherent order for budgets

app/models/budget.rb

@@ -33,7 +33,7 @@ class Budget < ActiveRecord::Base
   scope :open, -> { where.not(phase: "finished") }
 
   def self.current
-    where.not(phase: "drafting").last
+    where.not(phase: "drafting").order(:created_at).last
   end
 
   def description