[5.0] Allow S3 region in config/filesystems.php

Some AWS regions, such as eu-central-1 (Frankfurt) do not support the default "v2" authentication signature. Setting the region when creating the client enables the correct signature to be used. Requires aws-sdk-php >= 2.7.5
AWS SDK issue: https://github.com/aws/aws-sdk-php/issues/393
OH NO. We NEED an upper bound on the dependency. This is very bad...