Remove the dask pin introduced by #3127
Assuming they have a new release by then (should be dask 0.18.3 or later)
Good to go : dask v 0.19 is now out, and fixes it

To check : the key failing test was
python lib/iris/tests/unit/cube/test_Cube.py -v Test_data_dtype_fillvalue.test_lazydata___getitem__dtype

UPDATE: dask 0.19 fixes original, but seems to cause new problems with Iris tests 😞
See : #3159 and its checks