Fixed AmazonS3->files() implementation

I'm not sure if it ever worked before, but currently the implementation is broken: there's no `listKeys()` method on `S3Client` class.

This PR fixes the implementation to use the high-level `getIterator('ListObjects')` iterator, as documented on AWS SDK for PHP [here](http://docs.aws.amazon.com/AmazonS3/latest/dev/ListingObjectKeysUsingPHP.html) and [here](http://docs.aws.amazon.com/aws-sdk-php/latest/class-Aws.S3.S3Client.html#_listObjects).