Use conda-forge when testing python 3.6 (not 3.5)
Use conda-forge when testing python 3.6 (not 3.5)

Conda-forge stopped building Python 3.5 packages in October 2018. This
may be the cause of mixed-channel environments failing tests in 3.5.

Also configure conda with latest installer, not to prompt to continue.

tomalrussell/smif,nismod/smif,tomalrussell/smif,willu47/smif,willu47/smif,willu47/smif,nismod/smif,nismod/smif,tomalrussell/smif,willu47/smif,tomalrussell/smif,nismod/smif