Move singleton requirement to the config class itself since it is the only class which uses the Singleton module.

lib/paper_trail.rb

@@ -1,4 +1,3 @@
-require 'singleton'
 require 'yaml'
 
 require 'paper_trail/config'

lib/paper_trail/config.rb

@@ -1,3 +1,5 @@
+require 'singleton'
+
 module PaperTrail
   class Config
     include Singleton