Fix S3 url building, fixes #3

[madmuffin1]

According to the documentation, the S3 URLs must have a different form than this template used. At least for me, in eu-central-1 region the original TemplateURL style did not work.

See http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-stack.html#TemplateURL