XRay exporter should use `AWS_REGION` if present before defaulting to `us-east-1`

[JeremieRodon]

I suppose everything is in the title.

I believe that most of the time, if somebody uses the XRay exporter of the Rotel Lambda layer, they will expect it to default to the region in which the Lambda is deployed.

If no command-line argument (or the equivalent env var) is passed, how would you feel about having Rotel defaulting to whatever is in AWS_REGION (if present), or AWS_DEFAULT_REGION (if present), or 'us-east-1`?

I could work on the subject and do a PR if you'd like.

[JeremieRodon]

Proposed changes:
#255