Add 4.0 in CI #27

Job	Run time
test (ubuntu-latest, 2.7)	22s
test (ubuntu-latest, 2.6)	27s
test (ubuntu-latest, 3.2)	11s
test (ubuntu-latest, 3.1)	9s
test (ubuntu-latest, 3.4)	13s
test (ubuntu-latest, 4.0)	11s
test (windows-latest, 2.6)	1m 43s
test (ubuntu-latest, jruby)	44s
test (windows-latest, 3.0)	57s
test (windows-latest, 3.2)	51s
test (windows-latest, 3.3)	51s
test (ubuntu-latest, 3.3)	9s
test (ubuntu-latest, 3.0)	9s
test (windows-latest, 3.4)	48s
test (ubuntu-latest, truffleruby)	24s
test (windows-latest, 4.0)	14s
test (windows-latest, 3.1)	1m 3s
test (windows-latest, 2.7)	1m 35s
	11m 1s

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