Add IidRoutes to Epic model

0af2ab18 decoubled the `to_param`
behavior from `AtomicInternalId`, but this model was forgotten.

ee/app/models/ee/epic.rb

@@ -4,6 +4,7 @@ module EE
 
     prepended do
       include AtomicInternalId
+      include IidRoutes
       include ::Issuable
       include Noteable
       include Referable