Allow use of amzn-ship in non-default regions

The aws-sdk defaults to `us-east-1` if no region is specified. This is
great, unless you're elsewhere...

In addition, the SDK implicitly uses the standard AWS_ACCESS_KEY_ID and
AWS_SECRET_ACCESS_KEY so I've taken the liberty of removing them from
the .new calls.
Merge pull request #1 from prognostikos/allow-non-default-regions

Allow use of amzn-ship in non-default regions