Warn user when local version of Node is greater than 6.9.1

[beaufortfrancois]

As Cloud functions only work with Node.js 6.9.1 for now, it would be great if we could add a warning in the console when local version of Node.js is superior to 6.9.1.

Steps to reproduce:

1. I'm playing with async/await in a recent version of Node.js.
2. It runs great locally!
3. I deploy it successfully
4. I run it and it fails ;(

[jmdobry]

Good idea. It does currently verify that your version of Node.js is at least 6.9.1, so it will be easy enough to print the warning when you're higher than 6.9.1.