Use Rails 5 conventions in ballot migrations

These migrations and models were added after the Rails 5 branch was
created but before it was merged.

app/models/poll/ballot.rb

@@ -1,4 +1,4 @@
-class Poll::Ballot < ActiveRecord::Base
+class Poll::Ballot < ApplicationRecord
   belongs_to :ballot_sheet, class_name: Poll::BallotSheet
 
   validates :ballot_sheet_id, presence: true

app/models/poll/ballot_sheet.rb

@@ -1,4 +1,4 @@
-class Poll::BallotSheet < ActiveRecord::Base
+class Poll::BallotSheet < ApplicationRecord
   belongs_to :poll
   belongs_to :officer_assignment
   has_many :ballots, class_name: Poll::Ballot

db/migrate/20170514192303_add_balloted_heading_id_to_users.rb

@@ -1,4 +1,4 @@
-class AddBallotedHeadingIdToUsers < ActiveRecord::Migration
+class AddBallotedHeadingIdToUsers < ActiveRecord::Migration[4.2]
   def change
     add_column :users, :balloted_heading_id, :integer, default: nil
   end

db/migrate/20180604173248_add_budget_to_polls.rb

@@ -1,4 +1,4 @@
-class AddBudgetToPolls < ActiveRecord::Migration
+class AddBudgetToPolls < ActiveRecord::Migration[4.2]
   def change
     add_reference :polls, :budget, index: { unique: true }, foreign_key: true
   end

db/migrate/20180621182723_create_poll_ballot_sheets.rb

@@ -1,4 +1,4 @@
-class CreatePollBallotSheets < ActiveRecord::Migration
+class CreatePollBallotSheets < ActiveRecord::Migration[4.2]
   def change
     create_table :poll_ballot_sheets do |t|
       t.text :data

db/migrate/20180704093831_create_poll_ballot.rb

@@ -1,4 +1,4 @@
-class CreatePollBallot < ActiveRecord::Migration
+class CreatePollBallot < ActiveRecord::Migration[4.2]
   def change
     create_table :poll_ballots do |t|
       t.integer :ballot_sheet_id

db/migrate/20180704095538_add_physical_to_budget_ballot.rb

@@ -1,4 +1,4 @@
-class AddPhysicalToBudgetBallot < ActiveRecord::Migration
+class AddPhysicalToBudgetBallot < ActiveRecord::Migration[4.2]
   def change
     add_column :budget_ballots, :physical, :boolean, default: false
     add_column :budget_ballots, :poll_ballot_id, :integer