fixup! Run Ruby 2.2 on Ubuntu 20.04 #205

Job	Run time
rails-3-4-5-maze-runner (2.0, 3) / maze-runner	9m 22s
rails-3-4-5-maze-runner (2.2, 4) / maze-runner	9m 8s
rails-3-4-5-maze-runner (2.2, 5) / maze-runner	11m 5s
rails-3-4-5-maze-runner (2.5, 4) / maze-runner	14m 10s
rails-3-4-5-maze-runner (2.5, 3) / maze-runner	8m 17s
rails-3-4-5-maze-runner (2.6, 5) / maze-runner	14m 10s
	1h 6m 12s

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