Notable changes:
* backend/s3: Automatic validation of `eu-north-1` region
* backend/s3: Support for `credential_process` handling in AWS configuration file
Updated via:
```
go get github.com/aws/aws-sdk-go@v1.16.4
go get github.com/terraform-providers/terraform-provider-aws@v1.52.0
go mod tidy
go mod vendor
```
* Remove `make updatedeps` from Travis build. We'll follow up with more
specific plans around dependency updating in subsequent PRs.
* Update all `make` targets to set `GO15VENDOREXPERIMENT=1` and to
filter out `/vendor/` from `./...` where appropriate.
* Temporarily remove `vet` from the `make test` target until we can
figure out how to get it to not vet `vendor/`. (Initial
experimentation failed to yield the proper incantation.)
Everything is pinned to current master, with the exception of:
* Azure/azure-sdk-for-go which is pinned before the breaking change today
* aws/aws-sdk-go which is pinned to the most recent tag
The documentation still needs to be updated, which we can do in a follow
up PR. The goal here is to unblock release.
Brian Flad