Fix s3 signature by sorting x-amz headers by their key

To create the signature, headers need to be sorted by their key, not key:value
See https://github.com/aws/aws-sdk-js/blob/master/lib/signers/s3.js#L82

Change to github.com/atlassian/goamz/

Revert "Change to github.com/atlassian/goamz/"

This reverts commit ecf709033f2500522d979ef1fcadbba1697bf4f6.
Merge pull request #215 from atlassian/fix-s3-signature

Fix s3 signature by sorting x-amz headers by their key