Add version 4.2 to all existing migrations

DB schema generated by a migration may look different in
rails 4 and 5 (because rails 5 may use different default values).
For this reason it's important to explicitly set for which rails
version a migration was written for.

See https://stackoverflow.com/questions/35929869/activerecordmigration-deprecation-warning-asks-for-rails-version-but-im-no/35930912#35930912

db/migrate/20140313092127_init_schema.rb

-class InitSchema < ActiveRecord::Migration
+class InitSchema < ActiveRecord::Migration[4.2]
   DOWNTIME = true
 
   # rubocop:disable Metrics/AbcSize

db/migrate/20140407135544_fix_namespaces.rb

-class FixNamespaces < ActiveRecord::Migration
+class FixNamespaces < ActiveRecord::Migration[4.2]
   DOWNTIME = false
 
   def up

db/migrate/20140414131055_change_state_to_allow_empty_merge_request_diffs.rb

 # rubocop:disable all
-class ChangeStateToAllowEmptyMergeRequestDiffs < ActiveRecord::Migration
+class ChangeStateToAllowEmptyMergeRequestDiffs < ActiveRecord::Migration[4.2]
   def up
     change_column :merge_request_diffs, :state, :string, null: true,
                   default: nil

db/migrate/20140416074002_add_index_on_iid.rb

 # rubocop:disable all
-class AddIndexOnIid < ActiveRecord::Migration
+class AddIndexOnIid < ActiveRecord::Migration[4.2]
   def change
     RemoveDuplicateIid.clean(Issue)
     RemoveDuplicateIid.clean(MergeRequest, 'target_project_id')

db/migrate/20140416185734_index_on_current_sign_in_at.rb

 # rubocop:disable all
-class IndexOnCurrentSignInAt < ActiveRecord::Migration
+class IndexOnCurrentSignInAt < ActiveRecord::Migration[4.2]
   def change
     add_index :users, :current_sign_in_at
   end

db/migrate/20140428105831_add_notes_index_updated_at.rb

 # rubocop:disable all
-class AddNotesIndexUpdatedAt < ActiveRecord::Migration
+class AddNotesIndexUpdatedAt < ActiveRecord::Migration[4.2]
   def change
     add_index :notes, :updated_at
   end

db/migrate/20140502115131_add_repo_size_to_db.rb

 # rubocop:disable all
-class AddRepoSizeToDb < ActiveRecord::Migration
+class AddRepoSizeToDb < ActiveRecord::Migration[4.2]
   def change
     add_column :projects, :repository_size, :float, default: 0
   end

db/migrate/20140502125220_migrate_repo_size.rb

 # rubocop:disable all
-class MigrateRepoSize < ActiveRecord::Migration
+class MigrateRepoSize < ActiveRecord::Migration[4.2]
   DOWNTIME = false
 
   def up

db/migrate/20140611135229_add_position_to_merge_request.rb

 # rubocop:disable all
-class AddPositionToMergeRequest < ActiveRecord::Migration
+class AddPositionToMergeRequest < ActiveRecord::Migration[4.2]
   def change
     add_column :merge_requests, :position, :integer, default: 0
   end

db/migrate/20140625115202_create_users_star_projects.rb

 # rubocop:disable all
-class CreateUsersStarProjects < ActiveRecord::Migration
+class CreateUsersStarProjects < ActiveRecord::Migration[4.2]
   DOWNTIME = false
 
   def change

db/migrate/20140729134820_create_labels.rb

 # rubocop:disable all
-class CreateLabels < ActiveRecord::Migration
+class CreateLabels < ActiveRecord::Migration[4.2]
   DOWNTIME = false
 
   def change

db/migrate/20140729140420_create_label_links.rb

 # rubocop:disable all
-class CreateLabelLinks < ActiveRecord::Migration
+class CreateLabelLinks < ActiveRecord::Migration[4.2]
   DOWNTIME = false
 
   def change

db/migrate/20140729145339_migrate_project_tags.rb

-class MigrateProjectTags < ActiveRecord::Migration
+class MigrateProjectTags < ActiveRecord::Migration[4.2]
   def up
     ActsAsTaggableOn::Tagging.where(taggable_type: 'Project', context: 'labels').update_all(context: 'tags')
   end

db/migrate/20140729152420_migrate_taggable_labels.rb

 # rubocop:disable all
-class MigrateTaggableLabels < ActiveRecord::Migration
+class MigrateTaggableLabels < ActiveRecord::Migration[4.2]
   def up
     taggings = ActsAsTaggableOn::Tagging.where(taggable_type: ['Issue', 'MergeRequest'], context: 'labels')
     taggings.find_each(batch_size: 500) do |tagging|

db/migrate/20140730111702_add_index_to_labels.rb

 # rubocop:disable all
-class AddIndexToLabels < ActiveRecord::Migration
+class AddIndexToLabels < ActiveRecord::Migration[4.2]
   def change
     add_index "labels", :project_id
     add_index "label_links", :label_id

db/migrate/20140903115954_migrate_to_new_shell.rb

 # rubocop:disable all
-class MigrateToNewShell < ActiveRecord::Migration
+class MigrateToNewShell < ActiveRecord::Migration[4.2]
   def change
     return if Rails.env.test?
 

db/migrate/20140907220153_serialize_service_properties.rb

 # rubocop:disable all
-class SerializeServiceProperties < ActiveRecord::Migration
+class SerializeServiceProperties < ActiveRecord::Migration[4.2]
   def change
     unless column_exists?(:services, :properties)
       add_column :services, :properties, :text

db/migrate/20140914113604_add_members_table.rb

 # rubocop:disable all
-class AddMembersTable < ActiveRecord::Migration
+class AddMembersTable < ActiveRecord::Migration[4.2]
   DOWNTIME = false
 
   def change

db/migrate/20140914145549_migrate_to_new_members_model.rb

 # rubocop:disable all
-class MigrateToNewMembersModel < ActiveRecord::Migration
+class MigrateToNewMembersModel < ActiveRecord::Migration[4.2]
   def up
     execute "INSERT INTO members ( user_id, source_id, source_type, access_level, notification_level, type ) SELECT user_id, group_id, 'Namespace', group_access, notification_level, 'GroupMember' FROM users_groups"
     execute "INSERT INTO members ( user_id, source_id, source_type, access_level, notification_level, type ) SELECT user_id, project_id, 'Project', project_access, notification_level, 'ProjectMember' FROM users_projects"

db/migrate/20140914173417_remove_old_member_tables.rb

 # rubocop:disable all
-class RemoveOldMemberTables < ActiveRecord::Migration
+class RemoveOldMemberTables < ActiveRecord::Migration[4.2]
   DOWNTIME = false
 
   def up

db/migrate/20141006143943_move_slack_service_to_webhook.rb

 # rubocop:disable all
-class MoveSlackServiceToWebhook < ActiveRecord::Migration
+class MoveSlackServiceToWebhook < ActiveRecord::Migration[4.2]
 
   DOWNTIME = true
   DOWNTIME_REASON = 'Move old fields "token" and "subdomain" to one single field "webhook"'

db/migrate/20141007100818_add_visibility_level_to_snippet.rb

 # rubocop:disable all
-class AddVisibilityLevelToSnippet