rubocop noise: fixes FrozenStringLiteralComment

Adding the .ruby-verison file triggered previously un-run cops, specifically:

  This cop is designed to help upgrade to Ruby 3.0. It will add the
  comment `# frozen_string_literal: true` to the top of files to enable
  frozen string literals. Frozen string literals will be default in Ruby
  3.0. The comment will be added below a shebang and encoding comment. The
  frozen string literal comment is only valid in Ruby 2.3+.

More info on rubocop [Automatic-Corrections](https://github.com/bbatsov/rubocop/wiki/Automatic-Corrections)

db/migrate/20160726193255_db_init.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 class DbInit < ActiveRecord::Migration[5.0]
   def change
     create_table :candidates do |t|

db/migrate/20160727154057_add_quiz_to_candidate.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 class AddQuizToCandidate < ActiveRecord::Migration[5.0]
   def change
     add_column :candidates, :quiz_id, :integer

db/migrate/20160803003932_add_attachments_to_questions.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 class AddAttachmentsToQuestions < ActiveRecord::Migration[5.0]
   def change
     add_column :questions, :attachment, :string

db/migrate/20160818225721_add_name_to_quiz.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 class AddNameToQuiz < ActiveRecord::Migration[5.0]
   def change
     add_column :quizzes, :name, :string, after: :id

db/migrate/20160824183159_add_resets_to_users.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 class AddResetsToUsers < ActiveRecord::Migration[5.0]
   def change
     add_column :users, :reset_token, :string

db/migrate/20160826200610_encode_candidate_emails.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 class EncodeCandidateEmails < ActiveRecord::Migration[5.0]
   def change
     sql = "select id, email from candidates;"