add minimum Ruby version to gemspec

[tilsammans]

This helps dependency resolve. Or is there a reason it isn't there?

If you want, I can also add ruby version requirements to any stable branches.

[gongo]

Hi @tilsammans
Thanks for good pull request. But I will not merge that change.

Because, turnip don't support Ruby 2.1 or lower. But still work .
I don't want to fix version yet for who can not use 2.2.x or higher for some reason. 🙏

[tilsammans]

Ah, got it. It's about support and guarantees. I understand!