Add support for configuring AWS S3 region.

AWS S3 buckets that are created using a non-default region does need to set `region` key on `S3Client::factory()` call.

Not configuring this way throws a `S3Exception` as follows:

<pre>The bucket you are attempting to access must be addressed using the specified endpoint. Please send all future requests to this endpoint: "...".</pre>

I'll be pushing a new PR to `laravel/laravel` develop branch to fulfill this one.