fix rack deprecation warning (#1142) #270

Job	Run time
build (2.5.3, gemfiles/rails50.gemfile)	58s
build (2.5.3, gemfiles/rails51.gemfile)	57s
build (2.5.3, gemfiles/rails52.gemfile)	1m 1s
build (2.5.3, gemfiles/rails60.gemfile)	3m 1s
build (2.5.3, gemfiles/rails61.gemfile)	1m 58s
build (2.6.0, gemfiles/rails50.gemfile)	1m 1s
build (2.6.0, gemfiles/rails51.gemfile)	1m 0s
build (2.6.0, gemfiles/rails52.gemfile)	1m 8s
build (2.6.0, gemfiles/rails60.gemfile)	2m 4s
build (2.6.0, gemfiles/rails61.gemfile)	1m 56s
build (2.6.6, gemfiles/rails50.gemfile)	1m 6s
build (2.6.6, gemfiles/rails51.gemfile)	1m 1s
build (2.6.6, gemfiles/rails52.gemfile)	1m 15s
build (2.6.6, gemfiles/rails60.gemfile)	2m 7s
build (2.6.6, gemfiles/rails61.gemfile)	1m 57s
build (2.7.2, gemfiles/rails50.gemfile)	1m 9s
build (2.7.2, gemfiles/rails51.gemfile)	59s
build (2.7.2, gemfiles/rails52.gemfile)	1m 11s
build (2.7.2, gemfiles/rails60.gemfile)	2m 7s
build (2.7.2, gemfiles/rails61.gemfile)	2m 3s
build (gemfiles/rails71.gemfile, 3.2.2)	45s
build (gemfiles/rails71.gemfile, 3.1.1)	48s
build (gemfiles/rails70.gemfile, 3.1.1)	46s
build (gemfiles/rails70.gemfile, 3.0.3)	39s
build (gemfiles/rails61.gemfile, 3.0.3)	40s
	33m 37s

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