Retrieve runtime value for region

[imranismail]

{:system, "AWS_DEFAULT_REGION"} wont work because this line of code doesn't retrieve the runtime value.

https://github.com/ex-aws/ex_aws/blob/59f5dea825f92b89051db8f8308bb8a9568de444/lib/ex_aws/config.ex#L37

Suggested fix:

region = retrieve_runtime_value(region, nil)

If this is right, let me know and I'll open a PR

[joffotron]

Related to the discussion at https://github.com/ex-aws/ex_aws/issues/365