Update ensure_uuid method in Ahoy initializer

It's possible to use the `Ahoy::Tracker::UUID_NAMESPACE` since Ahoy
2.1.0 [1].

[1] https://github.com/ankane/ahoy/commit/44f7956bad

config/initializers/ahoy.rb

@@ -24,12 +24,10 @@ class Ahoy::Store < Ahoy::DatabaseStore
     Visit
   end
 
-  UUID_NAMESPACE = UUIDTools::UUID.parse("a82ae811-5011-45ab-a728-569df7499c5f")
-
   def ensure_uuid(id)
     UUIDTools::UUID.parse(id).to_s
   rescue
-    UUIDTools::UUID.sha1_create(UUID_NAMESPACE, id).to_s
+    UUIDTools::UUID.sha1_create(UUIDTools::UUID.parse(Ahoy::Tracker::UUID_NAMESPACE), id).to_s
   end
 
   def exclude?