Deploify, NPM module that creates a temporary stage environment of each PR

Hmm.. cool, I guess. Why though?

You're "configuring" your express server to use this package. Now you have a third party package right in the middle of your application for what benefit? I don't think it's very practical to run a new app for every PR. Sure, you have production tied to master and dev/staging tied to a staging branch, or tie master to staging and manually release production. However, what benefit does manual testing serve you on a per-PR basis?