FIX: Memoize Singed.output_directory to prevent repeated path

Author: benaitcheson

lib/singed/railtie.rb

@@ -14,7 +14,7 @@ class Railtie < Rails::Railtie
     end
 
     def self.init!
-      Singed.output_directory = Rails.root.join("tmp/speedscope")
+      Singed.output_directory ||= Rails.root.join("tmp/speedscope")
       Singed.backtrace_cleaner = Rails.backtrace_cleaner
     end
   end