Add version to migration files

These migrations were backported after upgrading to Rails 5.

db/migrate/20170628211547_add_ballot_line_counter_cached_to_budget_ballots.rb

@@ -1,4 +1,4 @@
-class AddBallotLineCounterCachedToBudgetBallots < ActiveRecord::Migration
+class AddBallotLineCounterCachedToBudgetBallots < ActiveRecord::Migration[4.2]
   def change
     add_column :budget_ballots, :ballot_lines_count, :integer, default: 0
   end

db/migrate/20171008154106_add_slug_to_polls.rb

@@ -1,4 +1,4 @@
-class AddSlugToPolls < ActiveRecord::Migration
+class AddSlugToPolls < ActiveRecord::Migration[4.2]
   def change
     add_column :polls, :slug, :string
   end

db/migrate/20181128175808_add_admin_to_documents.rb

@@ -1,4 +1,4 @@
-class AddAdminToDocuments < ActiveRecord::Migration
+class AddAdminToDocuments < ActiveRecord::Migration[4.2]
   def change
     add_column :documents, :admin, :boolean, default: false
   end

db/migrate/20190307165132_remove_question_and_external_url_from_proposals.rb

@@ -1,4 +1,4 @@
-class RemoveQuestionAndExternalUrlFromProposals < ActiveRecord::Migration
+class RemoveQuestionAndExternalUrlFromProposals < ActiveRecord::Migration[4.2]
   def change
     remove_column :proposals, :question, :string
     remove_column :proposals, :external_url, :string

db/migrate/20190307165337_remove_question_and_external_url_from_legislation_proposals.rb

@@ -1,4 +1,4 @@
-class RemoveQuestionAndExternalUrlFromLegislationProposals < ActiveRecord::Migration
+class RemoveQuestionAndExternalUrlFromLegislationProposals < ActiveRecord::Migration[4.2]
   def change
     remove_column :legislation_proposals, :question, :string
     remove_column :legislation_proposals, :external_url, :string