Exclude CRuby extension from JRuby gem #15

Job	Run time
ruby-versions / Generate Ruby versions	2s
test (2.5, ubuntu-latest)	42s
test (2.5, macos-latest)	1m 10s
test (2.5, windows-latest)	3m 17s
test (2.6, ubuntu-latest)	35s
test (2.6, macos-latest)	1m 35s
test (2.6, windows-latest)	3m 29s
test (2.7, ubuntu-latest)	35s
test (2.7, macos-latest)	1m 25s
test (2.7, windows-latest)	2m 48s
test (3.0, ubuntu-latest)	24s
test (3.0, macos-latest)	38s
test (3.0, windows-latest)	1m 57s
test (3.1, ubuntu-latest)	22s
test (3.1, macos-latest)	25s
test (3.1, windows-latest)	1m 18s
test (3.2, ubuntu-latest)	31s
test (3.2, macos-latest)	41s
test (3.2, windows-latest)	3m 53s
test (head, ubuntu-latest)	27s
test (head, macos-latest)	1m 14s
test (head, windows-latest)	2m 15s
test (jruby, ubuntu-latest)	5m 31s
test (jruby-head, ubuntu-latest)	5m 30s
test (truffleruby, ubuntu-latest)	2m 36s
test (truffleruby-head, ubuntu-latest)	3m 20s
	46m 40s

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