Merge branch 'sh-fix-milestone-id-migration' into 'master'

Fix missing WHERE clause in 20171106135924_issues_milestone_id_foreign_key migration

Closes gitlab-com/infrastructure#3416

See merge request gitlab-org/gitlab-ce!15997

db/migrate/20171106135924_issues_milestone_id_foreign_key.rb

@@ -16,6 +16,7 @@ class IssuesMilestoneIdForeignKey < ActiveRecord::Migration
 
     def self.with_orphaned_milestones
       where('NOT EXISTS (SELECT true FROM milestones WHERE milestones.id = issues.milestone_id)')
+        .where('milestone_id IS NOT NULL')
     end
   end