Add a flag to user in attempts model to make it optional

In rails 5, `belongs_to` adds the required validation by default with
it. Make it optional so that non-logged in users can also take the
survey.

app/models/rapidfire/attempt.rb

@@ -1,7 +1,12 @@
 module Rapidfire
   class Attempt < ActiveRecord::Base
     belongs_to :survey
-    belongs_to :user, polymorphic: true
     has_many   :answers, inverse_of: :attempt, autosave: true
+
+    if Rails::VERSION::MAJOR >= 5
+      belongs_to :user, polymorphic: true, optional: true
+    else
+      belongs_to :user, polymorphic: true
+    end
   end
 end