The test environment was using the file in `config/puma.rb`, meaning it
wouldn't work with Rails 5.1, which uses a different setup for Puma.
I've decided to create a new file called `defaults.rb`, which will be
used in every environment but development and test. We could also add an
empty file in `config/puma/test.rb`; I think that's less intuitive, but
it's a subjective opinion.
