Ignore existing Migration/Datetime cop offenders

db/migrate/20160518200441_add_artifacts_expire_date_to_ci_builds.rb

+# rubocop:disable Migration/Datetime
 class AddArtifactsExpireDateToCiBuilds < ActiveRecord::Migration
   def change
     add_column :ci_builds, :artifacts_expire_at, :timestamp

db/migrate/20160716115711_add_queued_at_to_ci_builds.rb

+# rubocop:disable Migration/Datetime
 class AddQueuedAtToCiBuilds < ActiveRecord::Migration
   include Gitlab::Database::MigrationHelpers

db/migrate/20170503021915_add_last_edited_at_and_last_edited_by_id_to_issues.rb

+# rubocop:disable Migration/Datetime
 # See http://doc.gitlab.com/ce/development/migration_style_guide.html
 # for more information on how to write migrations for GitLab.

db/migrate/20170503022548_add_last_edited_at_and_last_edited_by_id_to_merge_requests.rb

+# rubocop:disable Migration/Datetime
 # See http://doc.gitlab.com/ce/development/migration_style_guide.html
 # for more information on how to write migrations for GitLab.