Update minimum Ruby version in README #2330

Job	Run time
RuboCop	45s
specs (ubuntu, 4.0.1)	1m 39s
specs (macos, 3.3.10)	1m 45s
specs (ubuntu, 3.4.8)	1m 42s
specs (macos, 4.0.1)	1m 24s
specs (windows, 3.4.8)	3m 13s
specs (macos, 3.4.8)	2m 5s
specs (windows, 4.0.1)	3m 42s
specs (ubuntu, 4.0.1, --parser=parse.y)	1m 39s
specs (ubuntu, 4.0.1, LC_ALL=C)	1m 30s
specs (ubuntu, 4.0.1, --repeat 2)	2m 17s
specs (ubuntu, 3.3.10)	1m 35s
specs (windows, 3.3.10)	3m 34s
specs (ubuntu, 4.0.1, --enable-frozen-string-literal)	1m 29s
	28m 19s

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