Retry logic is broken when you provide a callback

[RobinClowers]

I just ran into a case where a push failed, but there were no retries. I looked into your code, and this line: https://github.com/mafintosh/sqs/blob/master/index.js#L71 makes it so that retry logic never happens if you provide a callback.

[GeoffreyPlitt]

@mafintosh Is this repo maintained?