Add "--list--updated--modules" cli option

One solution to this [common question](https://github.com/boxen/our-boxen/issues/275). Adds `--list-updated-modules` to the CLI to see what, if any, newer puppet module versions are available based on one's Puppetfile. Ensures Boxen's GitHub token will be temporarily set as `GITHUB_API_TOKEN` to not hit API limits.