v0.5.1 dbt_mailchimp_source

PR #17 includes the following updates:

🎉 Feature Update

• Updates the unique test for stg_mailchimp__members to include both member_id and list_id, as these make up the unique key for the respective source table

Under the Hood:

• Updates the seed data used in integration testing to include a new record for an existing member_id with a new list_id.

Full Changelog: v0.5.0...v0.5.1