Fix rubocop-rails version on CI
The projects use a fixed version of rubocop-rails, and the CI system currently installs the latest. This might introduce CI fails that are not reproducible locally without upgrading the rubocop dependency, as it did.
The fails introduced by release 2.4 of rubocop-rails were like this:
data-collector/spec/rails_helper.rb:11:5: C: Rails/FilePath: Please use Rails.root.join('path/to') instead.
Dir[Rails.root.join('spec', 'support', '**', '*.rb')].each { |f| require f }
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^