Update rubies to 2.6.4, 2.5.6 and 2.4.7

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[knu]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

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[dazuma]

Thanks @knu for the pull request! We need to build the binaries for the new Ruby versions before we can merge this. We'll do so in the next few days.

[knu]

Thank you!