Add missing rubocop configuration
Add missing rubocop configuration

This is specified in the developer documentation [1] but was missed in
the previous PR [2].

1: https://docs.publishing.service.gov.uk/manual/configure-linting.html#linting-ruby
2: https://github.com/alphagov/govuk_admin_template/pull/218

alphagov/govuk_admin_template,alphagov/govuk_admin_template,alphagov/govuk_admin_template