#40 fail to db:migrate
Merge pull request #42 from kakusuke/fix-migration

db:migrateに失敗
・update_historiesに於けるuser_idのnot null制約を解除。
・schema.rbを最新化
Revert "・共同編集機能実装。"

This reverts commit 02e346dc1d1e3b9fbea4825a9eb13edd32e4b6c1.

Conflicts:
	app/controllers/articles_controller.rb
	db/migrate/20140628151137_add_user_id_to_update_histories.rb
	db/schema.rb