Improve scripts/boot.sh and update README

I think we should recommend the way to start the server. It seems to me `scripts/boot.sh` should be that way.

This makes environment variables easier to manage without introducing new dependencies as discussed here: https://github.com/kytrinyx/exercism.io/pull/333

What do you think?

I've also mentioned `.ruby-version` here as people may not be aware of it when setting up for the first time.