Fix active job serializer railtie

Author: gmcgibbon

CHANGELOG.md

@@ -1,3 +1,5 @@
+*   Fix railtie logic that adds Active Job serialization support.
+
 ## Active Resource 5.1.0 (Nov 2, 2018) ##
 
 *   Improve support of Active Resource objects inside fibers.

lib/active_resource/railtie.rb

@@ -16,7 +16,7 @@ class Railtie < Rails::Railtie
     end
 
     initializer "active_resource.add_active_job_serializer" do |app|
-      if defined? app.config.active_job.custom_serializers
+      if app.config.try(:active_job)&.custom_serializers
         require "active_resource/active_job_serializer"
         app.config.active_job.custom_serializers << ActiveResource::ActiveJobSerializer
       end