Fix timezone conversion macro on redshift

[msnidal]

Hi there! First of all, thank you for building/maintaining this package, I am using it and it rocks.

When calling a calendar date macro (in my case dbt_date.today() on redshift with dbt 1.0.0, I received the following error:

18:26:50  Encountered an error:
Runtime Error
  Database Error in model util__dates (models/layer_1_cleaned/utils/util__dates.sql)
    syntax error at or near "UTC"
    LINE 18:  at time zone 'UTC' at time zone 'Etc/UTC' as 

I looked into it and basically it seems that the default implementation you had for the timezone conversion is the supported way in redshift, but dbt's dispatch was calling the postgres implementation per its search ordering, but the postgres implementation does not work in redshift.

I consulted the dbt docs to see the best way to handle this and they recommend implementing a redshift dispatch which simply calls the default implementation in cases like this (bottom of page) which seemed reasonable to me. Hence this PR! And thanks again for the package 😄

[clausherther]

Hi @msnidal, thanks for this! Good catch! We don't actually officially support or test on Redshift, but I think it makes sense to include this since it's an easy fix. I'll cut a release later today. 👍

[clausherther]

LGTM