Stop testing ruby 2.5 on macOS #506

Job	Run time
head-versions (ubuntu, jruby-head)	38s
other (ubuntu, truffleruby)	1m 30s
other (ubuntu, jruby-9.3)	43s
other (ubuntu, jruby-9.4)	49s
valgrind	4m 47s
head-versions (ubuntu, ruby-head)	29s
mri (ubuntu-latest, 2.7)	38s
mri (ubuntu-latest, 2.6)	36s
mri (ubuntu-latest, 3.1)	30s
mri (windows-latest, 3.2)	2m 13s
mri (ubuntu-latest, 2.5)	30s
mri (ubuntu-latest, 3.5)	29s
mri (macos-latest, 3.2)	33s
mri (macos-latest, 2.6)	57s
mri (ubuntu-latest, 3.0)	28s
mri (ubuntu-latest, 3.4)	36s
mri (ubuntu-latest, 4.0)	30s
mri (ubuntu-latest, 3.3)	29s
mri (windows-latest, 2.5)	2m 5s
mri (ubuntu-latest, 3.2)	32s
	20m 2s

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