Update dev-requirements.txt to use the dbt-adapters monorepo

[dbeatty10]

resolves #990

Problem

dbt-postgres moved to the dbt-adapters monorepo. So dbt_utils is now pointing to a stale version of dbt-postgres

See dbt-labs/dbt-adapters#615 for more detail.

Solution

Update the refs so that it points to dbt-postgres subdirectory within the dbt-adapters monorepo.

Checklist

• This code is associated with an issue which has been triaged and accepted for development.
• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue (except for #992)
• Tests are not required/relevant for this PR