stub Shard.relative_id_for

Change-Id: I00bec27dc2b9606f81d972fdb2942e0feff8aa8b
Reviewed-on: https://gerrit.instructure.com/17969
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
QA-Review: Jacob Fugal <jacob@instructure.com>

lib/stubs/shard.rb

@@ -62,6 +62,10 @@ class Shard
     any_id
   end
 
+  def self.relative_id_for(any_id, target_shard = nil)
+    any_id
+  end
+
   yaml_as "tag:instructure.com,2012:Shard"
 
   def self.yaml_new(klass, tag, val)