always return Postgres table names with schema to avoid awfulness

- without the schema, the migrations table gets truncated since the default AR method returns ‘public.schema_migrations’
- also a possibility is truncation of tables other than desired once since without the schema, Postgres has no way of knowing which table to truncate if two schemas have the same table
I'd prefer we didn't set an expectation on the internals of the class, but test the desired output state.