forbid buy when event tickets quantity is 0 or not enough quantity
Merge branch 'pr504'

Conflicts:
	app/controllers/event_orders_controller.rb
	app/models/event_order.rb
	app/views/events/show.html.slim
	db/schema.rb
	spec/features/event_orders_spec.rb
	spec/models/event_order_spec.rb
Merge pull request #507 from jasl/fix-friendly-id

更新friendly-id的调用