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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

15fcd965 (!18547) squashes old migrations
to a single one.

Old migrations had `rubocop:disable all` rule, but the new migration doesn't
have it, so the `downtime_check` fails.

This commit explicitly tells that the migration requires downtime.

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>