fixed outdated ruby versions listed in README #412

[coreytegeler]

Fixes #412

• tests pass (only changed README; I haven't properly setup mapknitter locally yet)
• code has been rebased on top of latest master (check if another pull request was added recently, and please rebase)
• pull request is descriptively named and, if possible, multiple commits squashed if they're smaller changes

[welcome]

Thanks for opening this pull request! Dangerbot will test out your code and reply in a bit with some pointers and requests.
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[welcome]

Congrats on merging your first pull request! 🙌🎉⚡️
Your code will likely be published to https://mapknitter.org in the next few days.
In the meantime, can you tell us your Twitter handle so we can thank you properly?
Now that you've completed this, you can help someone else take their first step!
See: Public Lab's coding community!

[jywarren]

Hooray! Great, thank you!!!!

[coreytegeler]

Thank you Jeffrey! Looking forward to contributing more now