Fixed missing default interval for Stream usage

[zygos]

I stumbled upon an issue where pino-cloudwatch as a Writable Stream would result in no reported events. That's because the default interval is only set when running through bin/pino-cloudwatch.js and not when it's directly imported. undefined interval would result in a failed this.interval > 0 check in chunky-stream resulting in skipped setTimeout and thus no reported events. Adding a default value fixed the issue.