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