Jupyter Notebook 5.1 release candidate

[Grant Nestor]

Hi everyone! 

We are planning to release notebook 5.1 this Thursday. The release candidate is available on PyPI. To try it out:

pip install notebook --pre --force-reinstall

Please give it a try and if you encounter any issues, please file an issue on Github. 

Thanks!

[Grant Nestor]

If the original command doesn't appear to download the 5.1.0rc1 version, then try adding the `--upgrade` flag: 

`pip install notebook --upgrade --pre --force-reinstall`.