pin dbt to <0.19.0 in dbt example

Summary: This likely points to a breakage in dagster-dbt, but currently only the dbt example tests are failing. More info here: https://github.com/dagster-io/dagster/issues/3616.

Test Plan: bk

Reviewers: bob, max, johann

Reviewed By: johann

Subscribers: johann

Differential Revision: https://dagster.phacility.com/D6189