Add removal of old function versions to `up prune`

[tj]

Lambda pulls in copies of your S3 zip files, and has a 75gb limit, so we have to remove old versions.

[skabbes]

I don't have code that i can plop right into the up project, but I did recently write a script to do just this. Hopefully it can save you, or someone some time (or frantic inability to deploy like it did me).

https://gist.github.com/skabbes/6cf76e8e9e91cd8767adcd1cd41b81e2

[tj]

@skabbes thanks! I swear I implemented this before, but it might be in a branch