fix bad path.join

lib/default-handlers.js

@@ -14,7 +14,7 @@ module.exports.setChallenge = function (args, challengePath, keyAuthorization, d
   // TODO should be args.webrootPath
   console.log('args.webrootPath, challengePath');
   console.log(args.webrootPath, challengePath);
-  mkdirp(path.join(args.webrootPath, challengePath), function (err) {
+  mkdirp(args.webrootPath, function (err) {
     if (err) {
       done(err);
       return;