So, Dependabot correctly reported that this version of puma is
vulernable, which I fixed in the main app already—but I didn't notice we
also use that version in this cute tiny placeholder app we use early in
the deployment process.
There's not a real security need to upgrade this, as this placeholder
app has no access to useful data when it is run, but I think it's better
to resolve this by fixing it than by silencing Dependabot! May as well!