Use strings for class_name

As mentioned in the Rails console:

DEPRECATION WARNING: Passing a class to the `class_name` is deprecated
and will raise an ArgumentError in Rails 5.2. It eagerloads more classes
than necessary and potentially creates circular dependencies. Please
pass the class name as a string.

app/models/poll/ballot.rb

@@ -1,5 +1,5 @@
 class Poll::Ballot < ApplicationRecord
-  belongs_to :ballot_sheet, class_name: Poll::BallotSheet
+  belongs_to :ballot_sheet, class_name: "Poll::BallotSheet"
 
   validates :ballot_sheet_id, presence: true
 

app/models/poll/ballot_sheet.rb

@@ -1,7 +1,7 @@
 class Poll::BallotSheet < ApplicationRecord
   belongs_to :poll
   belongs_to :officer_assignment
-  has_many :ballots, class_name: Poll::Ballot
+  has_many :ballots, class_name: "Poll::Ballot"
 
   validates :data, presence: true
   validates :poll_id, presence: true