Disable Rails/BulkChangeTable cop

Disable Rails/BulkChangeTable cop

If you have multiple alter commands in a migration (like adding two
columns to an existing table) this cop will suggest combining them using
`change_table :foo, :bulk => true`.

However, Strong Migrations can't inspect `change_table` blocks, and
therefore suggests using `safety_assured`, which mostly defeats the
purpose of using strong_migrations in the first place.

Instead, we should stick with the individual alter commands. Since
postgres runs DDL changes in a transaction anyway, there is not enough
benefit from the `change_table` approach to be worth losing the
safety net of the Strong Migrations checks.