Less Pure Copping: cop-out tmp folders for generated Rails apps

After having run the railties tests locally and running Rubocop, you'd
see lots of violations for the Rails apps generated in tmp folders.

Don't consider those violations as violating.

.rubocop.yml

@@ -8,6 +8,7 @@ AllCops:
   # to ignore them, so only the ones explicitly set in this file are enabled.
   DisabledByDefault: true
   Exclude:
+    - '**/tmp/**/*'
     - '**/templates/**/*'
     - '**/vendor/**/*'
     - 'actionpack/lib/action_dispatch/journey/parser.rb'