Fix rake db:seed_fu ENV=development

This problem was reported on https://gitlab.com/gitlab-org/cookbook-gitlab/issues/46

db/fixtures/development/07_milestones.rb

+ActiveRecord::Base.observers.disable(:milestone_observer)
+
 Milestone.seed(:id, [
   { id: 1,  project_id: 1, title: 'v' + Faker::Address.zip_code },
   { id: 2,  project_id: 1, title: 'v' + Faker::Address.zip_code },
@@ -16,3 +18,5 @@ Milestone.all.map do |ml|
   ml.set_iid
   ml.save
 end
+
+ActiveRecord::Base.observers.enable(:milestone_observer)