[Close #251] Relax gem version requirement #78

Job	Run time
test (2.5, gemfiles/rails_5_1.gemfile)	1m 32s
test (2.5, gemfiles/rails_5_2.gemfile)	1m 46s
test (2.5, gemfiles/rails_6_0.gemfile)	1m 47s
test (2.5, gemfiles/rails_6_1.gemfile)	1m 41s
test (2.6, gemfiles/rails_5_2.gemfile)	1m 41s
test (2.6, gemfiles/rails_6_0.gemfile)	1m 43s
test (2.6, gemfiles/rails_6_1.gemfile)	1m 43s
test (2.7, gemfiles/rails_5_1.gemfile)	1m 36s
test (2.7, gemfiles/rails_5_2.gemfile)	1m 40s
test (2.7, gemfiles/rails_6_0.gemfile)	1m 41s
test (2.7, gemfiles/rails_6_1.gemfile)	1m 42s
test (2.7, gemfiles/rails_7_0.gemfile)	1m 47s
test (2.7, gemfiles/rails_7_1.gemfile)	1m 45s
test (3.0, gemfiles/rails_6_0.gemfile)	1m 24s
test (3.0, gemfiles/rails_6_1.gemfile)	1m 26s
test (3.0, gemfiles/rails_7_0.gemfile)	1m 23s
test (3.0, gemfiles/rails_7_1.gemfile)	1m 31s
test (3.1, gemfiles/rails_6_0.gemfile)	1m 24s
test (3.1, gemfiles/rails_6_1.gemfile)	1m 20s
test (3.1, gemfiles/rails_7_0.gemfile)	1m 20s
test (3.1, gemfiles/rails_7_1.gemfile)	1m 25s
test (3.1, gemfiles/rails_7_2.gemfile)	1m 23s
test (3.2, gemfiles/rails_6_0.gemfile)	1m 21s
test (3.2, gemfiles/rails_6_1.gemfile)	1m 21s
test (3.2, gemfiles/rails_7_0.gemfile)	1m 25s
test (3.2, gemfiles/rails_7_1.gemfile)	1m 27s
test (3.2, gemfiles/rails_7_2.gemfile)	1m 32s
test (3.3, gemfiles/rails_6_0.gemfile)	1m 30s
test (3.3, gemfiles/rails_6_1.gemfile)	1m 34s
test (3.3, gemfiles/rails_7_0.gemfile)	1m 30s
test (3.3, gemfiles/rails_7_1.gemfile)	1m 30s
test (3.3, gemfiles/rails_7_2.gemfile)	1m 40s
test (head, gemfiles/rails_head.gemfile)	1m 35s
	51m 5s

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