autopkgtest must use installed library

--- a/spec/mongoid_spec.rb
+++ b/spec/mongoid_spec.rb
@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 require 'spec_helper'
-require File.expand_path('../lib/countries/mongoid', __dir__)
+require 'countries/mongoid'
 
 describe 'Mongoid support' do
   let(:britain) { ISO3166::Country.new('GB') }