Keep default puma settings in the test environment

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.

config/puma/defaults.rb

@@ -1,10 +1,9 @@
 #!/usr/bin/env puma
 
-rails_root = File.expand_path("../..", __FILE__)
+rails_root = File.expand_path("../../..", __FILE__)
 
 directory rails_root
 rackup "#{rails_root}/config.ru"
-environment "production"
 
 tag ""
 

config/puma/preproduction.rb

@@ -1,5 +1,5 @@
 #!/usr/bin/env puma
 
-_load_from File.expand_path("../../puma.rb", __FILE__)
+_load_from File.expand_path("../defaults.rb", __FILE__)
 
 environment "preproduction"

config/puma/production.rb

@@ -1,5 +1,5 @@
 #!/usr/bin/env puma
 
-_load_from File.expand_path("../../puma.rb", __FILE__)
+_load_from File.expand_path("../defaults.rb", __FILE__)
 
 environment "production"

config/puma/staging.rb

@@ -1,5 +1,5 @@
 #!/usr/bin/env puma
 
-_load_from File.expand_path("../../puma.rb", __FILE__)
+_load_from File.expand_path("../defaults.rb", __FILE__)
 
 environment "staging"