packages.yml refers to a deprecated version of dbt_utils

Context

fishtown-analytics/* packages have been renamed dbt-labs/*

Problem

Installing a package that depends on fishtown-analytics/dbt_utils creates duplicated package name error when using other packages that depend on dbt-labs/dbt_utils This creates packages incompatibility and difficulties to upgrade to dbt's latest version

Example

Upgrading to dbt 0.20.0 and installing these packages should create an error

packages:
  - package: dbt-labs/dbt_utils
    version: 0.7.0
  - package: dbt-labs/audit_helper
    version: 0.4.0
  - git: https://gitlab.com/gitlab-data/snowflake_spend.git
    revision: v1.2.0
  - package: dbt-labs/dbt_external_tables
    version: 0.7.0