Using the wrong cf-units from conda-forge

Turns out that we are using the old ``cf_units``, not the new ``cf-units`` package. https://github.com/SciTools/cf-units/issues/112 refers

Known places that this occurs:

 * https://github.com/SciTools/iris/blob/master/requirements/core.txt#L8
 