Upgrade to GitLab Styles 9

This contains upgrades RuboCop to 1.3.6 and RuboCop RSpec to 2.12.1.

Notable changes here are:

1. Cops now get their namespace from all but the last and the first two
   components, rather than simply the last component. See
   https://github.com/rubocop/rubocop/pull/8490
2. There is no more TopLevelDescribe helper for Rubocop RSpec, it's now
   TopLevelGroup. See https://github.com/rubocop/rubocop-rspec/pull/977
3. RuboCop Rails now complains if db/schema.rb doesn't contain a schema.