Use default_branch rather than master, feedback:

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5347#note_13173871

app/models/ci/pipeline.rb

@@ -21,7 +21,7 @@ module Ci
     after_save :keep_around_commits
 
     # ref can't be HEAD or SHA, can only be branch/tag name
-    scope :latest_successful_for, ->(ref) do
+    scope :latest_successful_for, ->(ref = default_branch) do
       where(ref: ref).success.order(id: :desc)
     end
 

app/models/project.rb

@@ -430,7 +430,7 @@ class Project < ActiveRecord::Base
   end
 
   # ref can't be HEAD, can only be branch/tag name or SHA
-  def latest_successful_builds_for(ref = 'master')
+  def latest_successful_builds_for(ref = default_branch)
     Ci::Build.joins(:pipeline).
       merge(pipelines.latest_successful_for(ref)).
       latest_successful_with_artifacts