Warn users when a the cli or an adapter is out of date

[jimthedev]

Now CLI does this and it is quite helpful. This is the file that makes it happen in now.

https://github.com/zeit/now-cli/blob/f0b0e23ce756c89f8c17ea011092bb5f82aaea64/lib/check-update.js

We could warn for either a cli out of date or adapter out of date.

[LinusU]

This would be awesome, I know there is a package that I've used multiple times, but the name escapes me at the moment 🤔

[LinusU]

This was the one I was thinking about :) https://github.com/yeoman/update-notifier

[jimthedev]

That looks pretty good. Seems like we could get this working for both the cli and adapters.