Add the retry for Route53 when PriorRequestNotComplete happens

[henrysher]

Fix #91 accroding to the document (http://docs.aws.amazon.com/Route53/latest/DeveloperGuide/DNSLimitations.html)

[jamesls]

Thanks. I'll also add the Throttling exception code from their docs as well:

If you submit more than five requests per second, Amazon Route 53 returns an HTTP 400 error (Bad request). The response header also includes a Code element with a value of Throttling and a Message element with a value of Rate exceeded.