Cannot be used with frankfurt aws region because of v4 signature

[andreialecu]

Knox does not support v4 signatures, which is needed for new aws regions.

See: Automattic/knox#254

There has been an open PR for 8 months for knox to fix it: Automattic/knox#273 but the maintainers seem to disagree with it.

As a suggestion, I would recommend using aws-sdk directly, which can handle v4 signatures, is better maintained and was made a lot easier to use recently.

[andreialecu]

I fixed this in #30.