[5.0] Added 'signature' and 'region' to AWS S3 config

Fix for the next errors: 
> "The authorization mechanism you have provided is not supported. Please use AWS4-HMAC-SHA256"

and

> "A region must be specified when using signature version 4"

The new Amazon regions support only Signature Version 4 in Request Authentication.
http://docs.aws.amazon.com/AmazonS3/latest/dev/UsingAWSSDK.html#specify-signature-version

Of course, you need to set these fields in the config of your app. But I didn't commit it, because it is not required for the all regions.

Also, this fix includes changes from https://github.com/laravel/framework/pull/5773