Check the specific method existance for Spring after_fork

There are gems that open Spring module to register new commands
so checking defined?(Spring) returns truthy value but in fact there is
no real Spring loaded, therefore there is exception when an application
is run without Spring.

Author: dck

lib/rails_semantic_logger/engine.rb

@@ -189,7 +189,7 @@ class Engine < ::Rails::Engine
       Resque.after_fork { |_job| ::SemanticLogger.reopen } if defined?(Resque)
 
       # Re-open appenders after Spring has forked a process
-      Spring.after_fork { |_job| ::SemanticLogger.reopen } if defined?(Spring)
+      Spring.after_fork { |_job| ::SemanticLogger.reopen } if defined?(Spring.after_fork)
     end
   end
 end