Warning about new version

[AuHau]

It is I think quite expected that swarm-cli will be installed into --global scope and it might be hard to keep track of what version of swarm-cli I am running and if there is not a new one out. I would maybe suggest having a check on every run that would check if a new version is not out and if so then display a warning about the new version (of course not in quite mode).

[ldeffenb]

I'll second this request having just been burned by an issue in 2.14.0 that was already fixed by 2.18.0. #550