Remove migrations warning

DEPRECATION WARNING: Directly inheriting from ActiveRecord::Migration
is deprecated. Please specify the Rails release the migration was
written for:

class MigrationClass < ActiveRecord::Migration[4.2]

(called from require at bin/rails:4)

db/migrate/20161221172447_add_selected_to_budget_investment.rb

@@ -1,4 +1,4 @@
-class AddSelectedToBudgetInvestment < ActiveRecord::Migration
+class AddSelectedToBudgetInvestment < ActiveRecord::Migration[4.2]
   def change
     add_column :budget_investments, :selected, :bool, default: false, index: true
   end