Move dev dependencies to Gemfile and fix versions #81

Job	Run time
test (ubuntu-latest, 2.7)	48s
test (ubuntu-latest, 3.0)	26s
test (ubuntu-latest, 3.1)	25s
test (ubuntu-latest, head)	1m 15s
test (ubuntu-latest, jruby)	1m 13s
test (ubuntu-latest, jruby-head)	1m 11s
test (ubuntu-latest, truffleruby)	2m 24s
test (ubuntu-latest, truffleruby-head)	2m 3s
test (macos-latest, 2.7)	2m 4s
test (macos-latest, 3.0)	47s
test (macos-latest, 3.1)	1m 7s
test (macos-latest, head)	3m 7s
test (macos-latest, jruby)	2m 54s
test (macos-latest, jruby-head)	1m 19s
test (macos-latest, truffleruby)	5m 52s
test (macos-latest, truffleruby-head)	2m 26s
	29m 21s

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