run migrations from all registered migrations paths

ActiveRecord::Migrator.migrate is supposed and able to receive an array of migration paths in order to support multiple directories containing migrations (e.g. inside engines...)

see https://github.com/rails/rails/blob/master/activerecord/lib/active_record/migration.rb#L782

since `rake db:migrate` finds and executes all containing migrations on the public schema (for pg), `rake apartment:migrate` should do the same on the tenants' schemas.