Retry logic will indefinitely retry

[nonken]

When hitting Dynamo throttling limits (429s), Vogels will indefinitely retry and never back out.

https://github.com/ryanfitz/vogels/blob/master/lib/batch.js#L45-L47

The AWS-SDK attaches a retryable property to the error object for 500s and 429s.
This behaviour can bring hosts down as async.doWhilst will just retry forever. I think it is better to not handle retries at all and let the SDK handle this.

See also: #63