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/20160524143107_add_heading_id_to_budget_ballot.rb

@@ -1,4 +1,4 @@
-class AddHeadingIdToBudgetBallot < ActiveRecord::Migration
+class AddHeadingIdToBudgetBallot < ActiveRecord::Migration[4.2]
   def change
     add_column :budget_ballots, :heading_id, :integer
     add_index :budget_ballots, :heading_id