Remove conda-forge from default install instructions (#2336)
Remove conda-forge from default install instructions (#2336)

* Remove conda-forge from default install instructions

Apparently people were confused by this, and thought that Dask was only
maintained on the conda-forge channel.  We change installation
instructions to explicitly state that Dask is available on either
defaults or conda-forge.  This removes the `-c conda-forge` flag from
the conda install line.

* simplify conda install language

mrocklin/dask,mraspaud/dask,mraspaud/dask,mrocklin/dask,ContinuumIO/dask,dask/dask,ContinuumIO/dask,blaze/dask,jakirkham/dask,jakirkham/dask,jcrist/dask,jcrist/dask,dask/dask,blaze/dask