Dask/cudf histogram fixes

[jonmmease]

This PR has two small fixes for displaying histograms using dask and cudf.

• First, the plotly histogram plot was passing Dask arrays into the Plotly Figure dictionary which plotly.py doesn't handle.
• Second, the histogram operation code was still using the deprecated normed argument in one place. Numpy still supports this with a runtime deprecation warning, but cupy doesn't. The replacement is the density kwarg, which we were already using several places in the same function so I don't think backward compatibility should be affected at all. Also, the normed kwarg has been deprecated in numpy for a long time now (since 1.6.0)

Do these make sense @philippjfr ?

[philippjfr]

I've got various cuDF fixes I still need to push up including a fix for normed and the Ibis PR completely refactors the histogram operation. So maybe drop the normed change for now.

[jonmmease]

Sure thing! Thanks for working on all that

[philippjfr]

Thanks @jonmmease!