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/20180613143922_add_moderation_attrs_to_investments.rb

@@ -1,4 +1,4 @@
-class AddModerationAttrsToInvestments < ActiveRecord::Migration
+class AddModerationAttrsToInvestments < ActiveRecord::Migration[4.2]
   def change
     change_table :budget_investments do |t|
       t.datetime :confirmed_hide_at