Updated nyc_taxi

[jbednar]

Updated the nyc_taxi example:

• Updated to current versions of HoloViz tools
• Fixed https://github.com/pyviz-topics/examples/issues/144 ; needed updating to match changes in HoloViews datashader support
• Switched away from broken tile sources (worked fine locally, but not on examples.pyviz.org or other deployments)
• Removed distracting ds.count('passenger_count') - confusingly seems to count the total passengers, but really only counts the number of records where that field is not missing
• Replaced ancient Hot colormap with colorcet.fire; Hot was better than other hot versions, but still not properly uniform
• Updated notebook and dashboard to use rasterize now that they are well supported in Bokeh

To do:

• [ ] Should update to Datashader 0.13 when released to get the dynspread fixes.
• [ ] Update last cell of nyc_taxi-nongeo.ipynb; currently breaks with error "No plotting class for Dataset found".
• [ ] Fix anaconda-project.yml to avoid dependencies/packages issue
• [ ] When above are done, run anaconda-project lock and update the nyc_taxi/anaconda-project-lock.yml file