Bump up 1.6.0 #135

Job	Run time
ruby-versions / Generate Ruby versions	4s
build (2.5 / ubuntu-latest)	28s
build (2.5 / macos-latest)	43s
build (2.5 / windows-latest)	2m 23s
build (2.6 / ubuntu-latest)	19s
build (2.6 / macos-latest)	52s
build (2.6 / windows-latest)	4m 26s
build (2.7 / ubuntu-latest)	19s
build (2.7 / macos-latest)	51s
build (2.7 / windows-latest)	1m 29s
build (3.0 / ubuntu-latest)	9s
build (3.0 / macos-latest)	14s
build (3.0 / windows-latest)	1m 25s
build (3.1 / ubuntu-latest)	12s
build (3.1 / macos-latest)	10s
build (3.1 / windows-latest)	1m 9s
build (3.2 / ubuntu-latest)	6s
build (3.2 / macos-latest)	16s
build (3.2 / windows-latest)	2m 20s
build (head / ubuntu-latest)	6s
build (head / macos-latest)	12s
build (head / windows-latest)	1m 30s
	19m 43s

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