You need to tell RuboCop to load the Packaging extension. There are three ways to do this:

RuboCop configuration file

Put this into your .rubocop.yml file:

require: rubocop-packaging

Alternatively, use the following array notation when specifying multiple extensions:

  - rubocop-other-extension
  - rubocop-packaging

Now you can run rubocop and it will automatically load the RuboCop Packaging cops together with the standard cops.

Command line

rubocop --require rubocop-packaging

Rake task do |task|
  task.requires << 'rubocop-packaging'