Get rid of rack warning
Get rid of rack warning

rack 2.1.0 introduces a deprecation warning
https://github.com/rack/rack/commit/626272b2bc6b270bbd3d2aa6aaa64722350f42be#diff-894804c816a11584eb28499cb8bbe396R6

that rails is triggering,
https://github.com/rails/rails/blob/1e14a2405b5e19b6758842dbd6d377c68d7f57ae/actionpack/lib/action_dispatch/middleware/static.rb#L20

but in later versions this warning will be removed
https://github.com/rack/rack/commit/f80e65d5dde251ee446e4c0bd038f8bc4ec30314#diff-894804c816a11584eb28499cb8bbe396L6

This warning was causing CI failures on rack 2.1.0:
rack 2.0.8: https://travis-ci.org/rspec/rspec-rails/jobs/635322909
rack 2.1.0: https://travis-ci.org/rspec/rspec-rails/jobs/635434487

rspec/rspec-rails,rspec/rspec-rails,rspec/rspec-rails