Update rubocop and rubocop-rspec to fix build errors
This fixes issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/25054 by upgrading both rubocop and rubocop-rspec. A change to CI was also required to remove the .rubocop_todo.yml require errors.
rubocop-rspec 1.9.1 supports Ruby 2.1 again. This support had been dropped from previous versions.