Ignore the Migration/Datetime cop in a migration that fix a column type to datetime_with_timezone

Signed-off-by: Rémy Coutable <remy@rymai.me>

db/post_migrate/20171221140220_schedule_issues_closed_at_type_change.rb

 # See http://doc.gitlab.com/ce/development/migration_style_guide.html
 # for more information on how to write migrations for GitLab.
-
+# rubocop:disable Migration/Datetime
 class ScheduleIssuesClosedAtTypeChange < ActiveRecord::Migration
   include Gitlab::Database::MigrationHelpers