don't raise when uniqueness contraints fail, as we already fail gracefully for these cases

lib/postzord/receiver/public.rb

@@ -45,7 +45,7 @@ class Postzord::Receiver::Public < Postzord::Receiver
   def save_object
     @object = Diaspora::Parser::from_xml(@salmon.parsed_data)
     raise "Object is not public" if object_can_be_public_and_it_is_not?
-    @object.save!
+    @object.save
   end
 
   # @return [Array<Integer>] User ids