Tests are failing on dask-dev

[dcherian]

Sample error from https://travis-ci.org/pydata/xarray/jobs/466431752

_______________________ test_dataarray_with_dask_coords ________________________
    def test_dataarray_with_dask_coords():
        import toolz
        x = xr.Variable('x', da.arange(8, chunks=(4,)))
        y = xr.Variable('y', da.arange(8, chunks=(4,)) * 2)
        data = da.random.random((8, 8), chunks=(4, 4)) + 1
        array = xr.DataArray(data, dims=['x', 'y'])
        array.coords['xx'] = x
        array.coords['yy'] = y
    
        assert dict(array.__dask_graph__()) == toolz.merge(data.__dask_graph__(),
                                                           x.__dask_graph__(),
                                                           y.__dask_graph__())
    
>       (array2,) = dask.compute(array)
xarray/tests/test_dask.py:824: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
../../../miniconda/envs/test_env/lib/python3.6/site-packages/dask/base.py:395: in compute
    dsk = collections_to_dsk(collections, optimize_graph, **kwargs)
../../../miniconda/envs/test_env/lib/python3.6/site-packages/dask/base.py:187: in collections_to_dsk
    for opt, val in groups.items()}
../../../miniconda/envs/test_env/lib/python3.6/site-packages/dask/base.py:187: in <dictcomp>
    for opt, val in groups.items()}
../../../miniconda/envs/test_env/lib/python3.6/site-packages/dask/base.py:212: in _extract_graph_and_keys
    graph = merge(*graphs)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
dicts = <dask.sharedict.ShareDict object at 0x7f307d29a128>, kwargs = {}
factory = <class 'dict'>, rv = {}
d = ('arange-36f53ab1e6153a63bbf7f4f8ff56693c', 0)
    def merge(*dicts, **kwargs):
        """ Merge a collection of dictionaries
    
        >>> merge({1: 'one'}, {2: 'two'})
        {1: 'one', 2: 'two'}
    
        Later dictionaries have precedence
    
        >>> merge({1: 2, 3: 4}, {3: 3, 4: 4})
        {1: 2, 3: 3, 4: 4}
    
        See Also:
            merge_with
        """
        if len(dicts) == 1 and not isinstance(dicts[0], dict):
            dicts = dicts[0]
        factory = _get_factory(merge, kwargs)
    
        rv = factory()
        for d in dicts:
>           rv.update(d)
E           ValueError: dictionary update sequence element #0 has length 39; 2 is required
../../../miniconda/envs/test_env/lib/python3.6/site-packages/toolz/dicttoolz.py:39: ValueError

[shoyer]

Yep, see dask/dask#4291 for the dask issue.

We should probably temporarily disable our dask-dev build on Travis.