Add support for AWS Credential Providers

Since version 1.5.3 of the AWS SDK it supports credential providers [1]. These patches allow two new cases of configuring the credentials for S3.

1) Via a `:credential_provider` that gets passed to the SDK.
2) Not set at all. In this case the SDK will automatically discover credentials.

I've also bumped the development dependency of aws-sdk to support these new feature.

[1] http://aws.amazon.com/releasenotes/Ruby/0304005114813505