is_s3_url will not work
as per [the amazon docs](http://docs.aws.amazon.com/AmazonS3/latest/dev/VirtualHosting.html#VirtualHostingCustomURLs)

> "Any bucket with a DNS compatible name may be referenced as follows: http://[BucketName].s3.amazonaws.com/[Filename], for example, http://images.johnsmith.net.s3.amazonaws.com/mydog.jpg. Using CNAME you can map images.johnsmith.net to an Amazon S3 host name so the previous URL could become: http://images.johnsmith.net/mydog.jpg."

this function is looking at the scheme which will never test as `'s3'` since that is not (to my knowledge) a valid URL protocol
