AWS is deprecating the Go 1.x Lambda engine, so users have to [migrate to a custom runtime on Amazon Linux 2](https://aws.amazon.com/blogs/compute/migrating-aws-lambda-functions-from-the-go1-x-runtime-to-the-custom-runtime-on-amazon-linux-2/). Shorter's SAM [template is using the deprecated Go 1.x engine](https://github.com/instedd/shorter/blob/6ad27873aa8f1c68a6af86ecd6ad3a2bd556ace4/template.yaml#L6). We should migrate that to the new engine.