Use sum, feedback:

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6084#note_14991392

lib/gitlab/ci/pipeline_duration.rb

@@ -133,9 +133,7 @@ module Gitlab
       end
 
       def process_duration(periods)
-        periods.inject(0) do |result, per|
-          result + per.duration
-        end
+        periods.sum(&:duration)
       end
     end
   end