We were using the `timestamps` method most of the time, but sometimes we
were creating the columns manually.
Note editing past migrations if fine as long as the SQL they generate
remains identical, which is the case here.
DEPRECATION WARNING: Directly inheriting from ActiveRecord::Migration
is deprecated. Please specify the Rails release the migration was
written for:
class MigrationClass < ActiveRecord::Migration[4.2]
(called from require at bin/rails:4)