learning_object is stageable

spec/factories/learning_objects.rb

 FactoryGirl.define do
-  factory :learning_object, aliases: [:complainable, :reviewable, :subjectable, :likeable, :collectionable, :bookmarkable] do
+  factory :learning_object, aliases: [:complainable, :reviewable, :stageable, :subjectable, :likeable, :collectionable, :bookmarkable] do
     sequence(:name) { |i| "LearningObject #{i}" }
     sequence(:id_dspace) { |i| i }
     publisher