add toolz as explicit dependency in installation docs

[alaniwi]

toolz is a dependency of dask, but only an "extras" dependency (per https://github.com/dask/dask/blob/master/setup.py). Although I am not familiar with dask, I assume from this it is possible to have an otherwise working version of dask without the extras, in which case toolz would not be needed -- but because iris uses dask.array, it is needed for iris.

Therefore it would be helpful if the iris installation documentation at http://scitools.org.uk/iris/docs/latest/installing.html explicitly listed toolz as one of the requirements.