bump min ci version to 2.4 #98

Job	Run time
ruby-versions / Generate Ruby versions	2s
build (2.4 / ubuntu-latest)	17s
build (2.4 / macos-latest)	1m 14s
build (2.5 / ubuntu-latest)	22s
build (2.5 / macos-latest)	45s
build (2.6 / ubuntu-latest)	19s
build (2.6 / macos-latest)	55s
build (2.7 / ubuntu-latest)	19s
build (2.7 / macos-latest)	40s
build (3.0 / ubuntu-latest)	5s
build (3.0 / macos-latest)	13s
build (3.1 / ubuntu-latest)	6s
build (3.1 / macos-latest)	12s
build (3.2 / ubuntu-latest)	9s
build (3.2 / macos-latest)	12s
build (3.3 / ubuntu-latest)	7s
build (3.3 / macos-latest)	14s
build (head / ubuntu-latest)	7s
build (head / macos-latest)	11s
build (jruby / ubuntu-latest)	34s
build (jruby / macos-latest)	45s
build (jruby-head / ubuntu-latest)	31s
build (jruby-head / macos-latest)	36s
build (truffleruby / ubuntu-latest)	27s
build (truffleruby / macos-latest)	35s
build (truffleruby-head / ubuntu-latest)	19s
build (truffleruby-head / macos-latest)	25s
	10m 41s

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