Change version of Debian used on CircleCI from buster to stretch for Go 1.13 builds

[leighmcculloch]

Change version of Debian used on CircleCI from buster to stretch for Go 1.13 builds, for consistency with the Go 1.12 builds.

In #1542 and #1696 we added Go 1.13 builds using Debian Buster because the official DockerHub images for Go didn't include a stretch variant.

In docker-library/golang#300 a stretch variant was accepted. Once it's gone through review and the image has been built and is available on DockerHub we should change the CircleCI config to use the stretch variant, and then remove the changes to the dependencies that are no longer required.