Bump knox version to support later versions of Node.

We were running into the issue outlined here https://github.com/LearnBoost/knox/issues/76 while using connect-assets-cdn. Upgrading to knox 0.8.x fixes the problem without introducing any apparent negative consequences.

The problem resulted in an error callback from S3Uploader.listExistingFiles, but the error is being ignored, so the problem results in an undefined variable error. I included an error guard clause in this pull patch to simplify future debugging.

Thanks for building this package. It's been very helpful on our project. - Alex