Add `logger` gem as a runtime dependency #270

Job	Run time
linting	13s
test (ubuntu, 2.7, false)	4m 31s
test (ubuntu, 3.0, false)	4m 18s
test (ubuntu, 3.1, false)	4m 21s
test (ubuntu, 3.2, false)	4m 19s
test (ubuntu, 3.3, false)	4m 18s
test (macos, 2.7, false)	4m 31s
test (macos, 3.0, false)	4m 17s
test (macos, 3.1, false)	4m 16s
test (macos, 3.2, false)	4m 15s
test (macos, 3.3, false)	4m 40s
test (macos, truffleruby-head, true)	5m 32s
test (ubuntu, truffleruby-head, true)	4m 49s
test (ubuntu, jruby, true)	4m 53s
test (ubuntu, head, true)	4m 17s
	1h 3m 30s

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