BUG: Fix unicode/str error when generating config file.

jupyterhub/app.py

@@ -9,6 +9,7 @@ import io
 import logging
 import os
 from datetime import datetime
+from six import text_type
 from subprocess import Popen
 
 try:
@@ -654,8 +655,8 @@ class JupyterHubApp(Application):
                 answer = ask()
             if answer.startswith('n'):
                 return
-        
-        config_text = self.generate_config_file()
+
+        config_text = text_type(self.generate_config_file(), 'utf8')
         print("Writing default config to: %s" % self.config_file)
         with io.open(self.config_file, encoding='utf8', mode='w') as f:
             f.write(config_text)