Fix frisp unable to get ARN.
Fix frisp unable to get ARN.

Possibly caused by AWS SDK change that now responds with "400" status
code, and the message says "Must specify userName when calling with
non-User credentials". Using STS is now the preferred way anyway.

silverstripeltd/frisp,silverstripeltd/frisp,silverstripe-platform/frisp,silverstripe-platform/frisp,silverstripeltd/frisp,silverstripe-platform/frisp