Update required Ruby version in README instructions

[coreytegeler]

Towards the bottom of the Prerequisites section of the README it is instructing to "install version 2.1.2 of Ruby" while MapKnitter 2 is currently using 2.4.4

Though this seems like an open-and-shut case, this is my first time attempting to contribute here so I would like a response to confirm this does indeed need to be fixed and I am not missing something. If so, I'd be happy to claim the issue myself and can fix right away and test my first pull request.

Thanks!

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[grvsachdeva]

hey @coreytegeler, nice catch and sure, go ahead. Thanks!

[coreytegeler]

Thank you Gaurav, I've just put in a pull request #416