fix from puts to use $stderr

Author: jercherng

source/lib/restacker.rb

@@ -278,7 +278,7 @@ def self.dump_stack_params(options)
       params = {}
       template["Parameters"].each do |key, value|
         params[key] = value['Default']
-        puts "WARNING: No default parameter set for #{key}." if value['Default'] == nil
+        $stderr.puts "WARNING: No default parameter set for #{key}." if value['Default'] == nil
       end
       puts params.to_yaml
     rescue JSON::ParserError => e