Move dev dependencies to Gemfile and fix versions #82

Job	Run time
test (ubuntu-latest, 2.7)	47s
test (ubuntu-latest, 3.0)	27s
test (ubuntu-latest, 3.1)	28s
test (ubuntu-latest, head)	1m 17s
test (ubuntu-latest, jruby)	1m 8s
test (ubuntu-latest, jruby-head)	1m 10s
test (ubuntu-latest, truffleruby)	40s
test (ubuntu-latest, truffleruby-head)	1m 56s
test (macos-latest, 2.7)	1m 32s
test (macos-latest, 3.0)	1m 11s
test (macos-latest, 3.1)	49s
test (macos-latest, head)	2m 51s
test (macos-latest, jruby)	1m 40s
test (macos-latest, jruby-head)	2m 56s
test (macos-latest, truffleruby)	7m 6s
test (macos-latest, truffleruby-head)	3m 49s
	29m 47s

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