Update repo to work with Terraform 0.13.x

[robmorgan]

1. Update CircleCI to use Terraform 0.13.x.
2. Update the version constraint on all examples to require at least Terraform 0.12.26.
3. Update pre-commit hooks to add gofmt.
4. Drop Go Dep in favor of Go modules.
5. Upgrade the tests to support Terratest v0.30.23

In general, TF 0.13.x seems backwards compatible from a testing perspective: I did not need to change anything in modules in this PR. I've also ensured the modules and examples work with Google Provider 3.x.

[robmorgan]

Waiting for an admin to re-enable the CircleCI tests.

[robmorgan]

Ok that's done, this PR is now ready for review!

[rhoboat]

LGTM. I checked the CI yaml and everything else looks good, too.

[robmorgan]

thanks @rhoboat !