Tutorial page on DOCS linked to old version

[vr00n]

The link to the tutorial on the docs page - http://agate.readthedocs.io/en/latest/tutorial.html links to here https://github.com/wireservice/agate/blob/1.5.6/tutorial.ipynb and 404s.

Needs to be updated to https://github.com/wireservice/agate/blob/master/tutorial.ipynb