LNX-330 - Check if inject_rum_script method exists

Author: homiedopie

lib/stackify/rum.rb

@@ -13,7 +13,7 @@ def initialize(config)
     end
 
     def insert_rum_script()
-      return StackifyRubyAPM.inject_rum_script if Rum.apm_loaded && defined?(StackifyRubyAPM)
+      return StackifyRubyAPM.inject_rum_script if Rum.apm_loaded && defined?(StackifyRubyAPM) && StackifyRubyAPM.respond_to?(:inject_rum_script)
 
       return '' unless @config