Added support for ENV['AWS_DEFAULT_REGION']

Support environment variable used by Amazon Command Line Interface.

The unified awscli uses the following environment variables:

* AWS_ACCESS_KEY_ID
* AWS_SECRET_ACCESS_KEY
* AWS_DEFAULT_REGION

The first two are supported by aws-sdk-ruby and this change adds support for the region.

This makes aws-sdk-ruby based scripts easier for users familiar with awscli.

http://docs.aws.amazon.com/cli/latest/userguide/cli-chap-getting-started.html#config-settings-and-precedence