Determine a mechanism for running bundix

Regardless of a user's workflow in development, we should try to ensure the `gemset.nix` is up-to-date for a given project before building or entering a shell.

For shells, maybe we can have a wrapper script/command that runs `nix-shell -p cacert bundix --run "bundix -l"`.

For deployments, I'm not sure what the best approach is.