make rubocop actually fail when problems were found

Rakefile

@@ -27,5 +27,4 @@ require 'rubocop/rake_task'
 desc 'Run RuboCop on the lib directory'
 RuboCop::RakeTask.new(:rubocop) do |task|
   task.patterns = ['bin/foreman-node', 'lib/**/*.rb', 'test/**/*.rb']
-  task.fail_on_error = false
 end