Code Quality Rubocop require not working

Hello,

We are using https://github.com/rubocop-hq/rubocop-rails in one of our projects, with

require: rubocop-rails

in the .rubocop.yml file, but the requirement is failing and the code quality step is failing.

Gem.gunzip called from /usr/local/bundle/gems/unicode-display_width-1.3.0/lib/unicode/display_width/index.rb:5.
/usr/local/lib/ruby/site_ruby/2.4.0/rubygems/core_ext/kernel_require.rb:117:in `require': cannot load such file -- rubocop-rails (LoadError)
	from /usr/local/lib/ruby/site_ruby/2.4.0/rubygems/core_ext/kernel_require.rb:117:in `require'

Any ideas? Using the Gitlab 11.5 config here: https://docs.gitlab.com/ee/user/project/merge_requests/code_quality.html#previous-job-definitions

A Gitlab EE customer.