use the same 'rubygems' require unless defined? Gem (as WarMain does)

Author: kares

lib/warbler/traits/jar.rb

@@ -29,7 +29,7 @@ def before_configure
       end
 
       def after_configure
-        config.init_contents << StringIO.new("require 'rubygems'\n")
+        config.init_contents << StringIO.new("require 'rubygems' unless defined?(Gem)\n")
       end
 
       def update_archive(jar)