List columns within the method (#11377)

To avoid the exception:

NoMethodError: undefined method `perform' for nil:NilClass
.../vendor/bundle/ruby/2.6.0/gems/strong_migrations-0.4.1/lib/strong_migrations/migration.rb:14:in `method_missing'
.../vendor/bundle/ruby/2.6.0/gems/activerecord-5.2.3/lib/active_record/migration.rb:604:in `method_missing'
.../db/migrate/20170918125918_ids_to_bigints.rb:69:in `<class:IdsToBigints>'
.../db/migrate/20170918125918_ids_to_bigints.rb:3:in `<top (required)>'