Fix date format for parameters CreatedAfter and CreatedBefore in Emr res...

When I call the describeJobFlows method with the CreatedAfter or CreatedBefore parameters, I always received an error from the server : class java.lang.String can not be converted to milliseconds since epoch
With previous version of the SDK, the error was : timestamp must follow ISO8601

After some debugging, Guzzle format the parameter to ISO 8601 (YYYY-MM-DDThh:mm:ssZ) but the API don't handle it.

If I change the format option on the resource file to "timestamp", everything is working.