Region name in arn patch is None

[Mike Fuller]

On some platforms (OSX) the ARN patch is populating the region part of the ARN with None. This is highly likely due to the hacky way we get the session object from the heap. We have an open PR with boto3 (https://github.com/boto/boto3/pull/898) to allow us to get access to the session object directly from the resource. Hopefully this will get merged.