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

Fixes a bug where doing `jupyterhub --generate-config` would generate the following stacktrace:

```
(jupyterhub)[~/clones/jupyterhub]@(master:b24df4f104)$ jupyterhub --generate-config
Overwrite jupyter_hub_config.py with default config? [y/N]y
Writing default config to: jupyter_hub_config.py
Traceback (most recent call last):
  File "/Users/ssanderson/.virtualenvs/jupyterhub/bin/jupyterhub", line 10, in <module>
    execfile(__file__)
  File "/Users/ssanderson/clones/jupyterhub/scripts/jupyterhub", line 4, in <module>
    main()
  File "/Users/ssanderson/clones/ipython/IPython/config/application.py", line 559, in launch_instance
    app.start()
  File "/Users/ssanderson/clones/jupyterhub/jupyterhub/app.py", line 686, in start
    self.write_config_file()
  File "/Users/ssanderson/clones/jupyterhub/jupyterhub/app.py", line 661, in write_config_file
    f.write(config_text)
TypeError: must be unicode, not str
```
It's possible that the real bug here is in `IPython.config.Application.generate_config`, which is not returning unicode when that appears to be the expectation here.  If so, feel free to disregard this PR.