Add channel/rubocop-1-21-0 (codeclimate#300)

* Update rubocop to 1.21.0

* Update scraped documentation

* Fix specs

Add channel/rubocop-1-20-0 (codeclimate#299)

* Update rubocop to 1.20.0

* Update scraped documentation

* Fix specs

Add channel/rubocop-1-20-0 (codeclimate#299)

* Update rubocop to 1.20.0

* Update scraped documentation

* Fix specs

Add channel/rubocop-1-20-0 (codeclimate#299)

* Update rubocop to 1.20.0

* Update scraped documentation

* Fix specs

Make channel 1-12-1 (mostly) Ruby 2.4 compatible (codeclimate#295)

- Update plugins to the last Ruby 2.4 compatible versions.
- Be specific about the versions in Gemfile, so bundle update works.
- Do the bundle update.
- Don't downgrade ruby-i18n and ruby-graphql; the listed versions are
  not compatible with Ruby 2.4, so don't use them if using Ruby 2.4.

[Fixes codeclimate#272, codeclimate#291, codeclimate#293]

Update channel 1-18-3 (codeclimate#296)

Update all plugins with bundle update; all Ruby 2.5-3.0 compatible.

[Fixes codeclimate#272, codeclimate#293]

[1-18-3] Update ruobocop-rails to 2.11.2 (codeclimate#294)

[1-12-1] Update ruobocop-rails to 2.9.1 (codeclimate#292)

Add channel/rubocop-1-18-3 (codeclimate#290)

* Update rubocop to v1.13.0

* Update scraped documentation

* Update rubocop to v1.14.0

* Update scraped documentation

* Update rubocop to v1.15.0

* Update scraped documentation

* Update rubocop to v1.16.0

* Update scraped documentation

* Update spec/support/config_upgrader_rubocop.yml

* Update rubocop to v1.16.1

* Update rubocop to v1.17.0

* Update scraped documentation

* Update rubocop to v1.18.2

* Update scraped documentation

* Update spec/support/config_upgrader_rubocop.yml

* Update rubocop to v1.18.3

* Update scraped documentation

Add channel/rubocop-1-12-1 (codeclimate#281)

* Update rubocop to v1.12.1

* Update scraped documentation