Migrated AmazonS3Resolver to version 2 of aws-sdk-php

I've updated the `AmazonS3Resolver` to work with version 2 of `aws-sdk-php`. This is a back-compatibility break as some configuration needs updating.

I could do with some help sorting out the unit tests as I don't really know what I'm doing there.

This fixes https://github.com/liip/LiipImagineBundle/issues/151