Land rapid7#6221, update references for caidao_php_backdoor_exec.rb

Author: wchen-r7

modules/exploits/multi/http/caidao_php_backdoor_exec.rb

@@ -23,7 +23,9 @@ def initialize(info = {})
       'References'        =>
         [
           ['URL', 'https://www.fireeye.com/blog/threat-research/2013/08/breaking-down-the-china-chopper-web-shell-part-i.html'],
-          ['URL', 'https://www.fireeye.com/blog/threat-research/2013/08/breaking-down-the-china-chopper-web-shell-part-ii.html']
+          ['URL', 'https://www.fireeye.com/blog/threat-research/2013/08/breaking-down-the-china-chopper-web-shell-part-ii.html'],
+          ['URL', 'https://www.exploit-db.com/docs/27654.pdf'],
+          ['URL', 'https://www.us-cert.gov/ncas/alerts/TA15-313A']
         ],
       'Platform'          => ['php'],
       'Arch'              => ARCH_PHP,