running closure_tree:migration gives: Directly inheriting from ActiveRecord::Migration is not supported.

[botp]

eg

$ bin/rails -v

Rails 5.1.6

$ gem query closure_tree

*** LOCAL GEMS ***

closure_tree (6.6.0)

$ bin/rails db:migrate

rails aborted!
StandardError: An error has occurred, this and all later migrations canceled:

Directly inheriting from ActiveRecord::Migration is not supported. Please specify the Rails release the migration was written for:

class CreateServiceClientDepartmentHierarchies < ActiveRecord::Migration

....
that should be ActiveRecord::Migration [5.1]