Use latest Ruby for frozen string literal test #388

Job	Run time
RuboCop	32s
specs (ubuntu, 3.1.6)	1m 38s
specs (ubuntu, 3.2.7)	1m 29s
specs (ubuntu, 3.3.7)	1m 39s
specs (ubuntu, 3.4.2)	1m 33s
specs (macos, 3.1.6)	1m 30s
specs (macos, 3.2.7)	1m 32s
specs (macos, 3.3.7)	1m 24s
specs (macos, 3.4.2)	1m 35s
specs (windows, 3.1.6)	4m 43s
specs (windows, 3.2.7)	4m 7s
specs (windows, 3.3.7)	4m 5s
specs (windows, 3.4.2)	3m 55s
specs (ubuntu, 3.4.2, --enable-frozen-string-literal)	1m 33s
	31m 15s

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