Adds dependent restricted association to some models

fix #348

app/models/difficulty_level.rb

@@ -2,7 +2,7 @@ class DifficultyLevel < ActiveRecord::Base
   attr_accessible :title, :description, :color, :conference_id
 
   belongs_to :conference
-  has_many :events
+  has_many :events, dependent: :nullify
 
   validates :title, presence: true
 end