rofl: Notify the user when an upgrade is available

[uniyalabhishek]

Because the CLI has the "latest" artifacts and a method to check whether an upgrade is possible. So it could just show something like the following after the build completes (so it doesn't get lost in all the output):

NOTE: A new version of artifacts is available. Run `oasis rofl upgrade` to upgrade.`

And then also we should change rofl upgrade so that it says something like the following (currently I think it doesn't output much):

Artifacts have been updated to the latest versions.
Run `oasis rofl build` to build with the new artifacts.

Or similar.