Support dbt v0.19.0

[jtcohen6]

• We've released dbt v0.19.0rc1, we should release dbt-spark v0.19.0rc1 for testing as well
• This may first require fixes to pytest-dbt-adapter (Not fully compatible with dbt-core 0.19.0rc1 dbt-adapter-tests#9)

[dbeatty10]

I don't currently have access to a Databricks version of Spark to test dbt snapshots against dbt-core 0.19.0rc1.

But I'm guessing that dbt/include/spark/macros/materializations/snapshot.sql will need to be updated like this:

    {% if not target_relation_exists %}

-       {% set build_sql = build_snapshot_table(strategy, model['injected_sql']) %}
+       {% set build_sql = build_snapshot_table(strategy, model['compiled_sql']) %}
        {% set final_sql = create_table_as(False, target_relation, build_sql) %}

    {% else %}