suggest shelley upgrades

Friend didn't know that Shelley could be upgraded, was hitting old bugs.

Shelley upgrades are not very discoverable. We should surface them more vigorously.

• Shelley can check for the existence of upgrades and give the ssh command to run
• When starting the repl, if any VMs that have ever run Shelley (do we know this?) have upgrade-able Shelleys, then we can add the upgrade hint to the "motd" suggested commands at the top

cc @philz