iris.coord_categorisation.add_weekday() fails with cftime v1.1.1

[TheElectricFlock]

iris.coord_categorisation.add_weekday() fails with cftime v1.1.1, the latest cftime release. Provided below is a traceback error.
>>> import iris
>>> import iris.coord_categorisation
>>> from glob import glob
>>> fnames = glob('/project/champ/data/cmip5/output1/ICHEC/EC-EARTH/historical/mon/atmos/Amon/r1i1p1/v20131231/tas/*.nc')
>>> cube = iris.load_cube(fnames[0])
>>> iris.coord_categorisation.add_weekday(cube, 'time', name='weekday')
Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/home/h01/jbedwell/miniconda/envs/test_environment/lib/python3.6/site-packages/iris/coord_categorisation.py", line 208, in add_weekday units='no_unit') File "/home/h01/jbedwell/miniconda/envs/test_environment/lib/python3.6/site-packages/iris/coord_categorisation.py", line 79, in add_categorised_coord result = category_function(from_coord, from_coord.points.ravel()[0]) File "/home/h01/jbedwell/miniconda/envs/test_environment/lib/python3.6/site-packages/iris/coord_categorisation.py", line 207, in <lambda> lambda coord, x: calendar.day_abbr[_pt_date(coord, x).weekday()], AttributeError: 'cftime._cftime.DatetimeGregorian' object has no attribute 'weekday'

[trexfeathers]

I can confirm this is replicable when running the test suite in Travis CI. Will investigate a fix (pinning or otherwise)