Dash Datashader does not install dask requirements

[shammamah-zz]

App name

dash-datashader

Description of bug

An install issue with dask:
(from the logs)

2019-08-19T16:11:26.712891563Z app[web.1]:     "fsspec is required to use any file-system functionality."
2019-08-19T16:11:26.712924421Z app[web.1]: ImportError: fsspec is required to use any file-system functionality. Please install using
2019-08-19T16:11:26.712929825Z app[web.1]: conda install -c conda-forge 'fsspec>=0.3.3'
2019-08-19T16:11:26.712934212Z app[web.1]: or
2019-08-19T16:11:26.712938098Z app[web.1]: pip install 'fsspec>=0.3.3'

Related issue in the dask repo here (it appears to be pretty recent): h2oai/db-benchmark#92

[lundstrj]

looks fixed.