Skipping / failing zarr tests

[rabernat]

Zarr tests are currently getting skipped on our main testing environments (because the zarr version is less than 2.2):
https://travis-ci.org/pydata/xarray/jobs/348350073#L1264

And failing in the py36-zarr-dev environment
https://travis-ci.org/pydata/xarray/jobs/348350087#L4989

I'm not sure how this regression occurred, but the zarr tests have been failing for a long time, e.g.
https://travis-ci.org/pydata/xarray/jobs/342651302

Possibly related to #1954

cc @jhamman

[rabernat]

The failure is

 ZarrDictStoreTest.test_explicitly_omit_fill_value_in_coord_via_encoding_kwarg _
self = <xarray.tests.test_backends.ZarrDictStoreTest testMethod=test_explicitly_omit_fill_value_in_coord_via_encoding_kwarg>
    def test_explicitly_omit_fill_value_in_coord_via_encoding_kwarg(self):
        ds = Dataset({'x': ('y', [np.pi, -np.pi])}, coords={'y': [0.0, 1.0]})
        kwargs = dict(encoding={'y': {'_FillValue': None}})
>       with self.roundtrip(ds, save_kwargs=kwargs) as actual:
xarray/tests/test_backends.py:721: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
../../../miniconda/envs/test_env/lib/python3.6/contextlib.py:81: in __enter__
    return next(self.gen)
xarray/tests/test_backends.py:1152: in roundtrip
    self.save(data, store_target, **save_kwargs)
xarray/tests/test_backends.py:1141: in save
    dataset.to_zarr(store=store_target, **kwargs)
xarray/core/dataset.py:1161: in to_zarr
    group=group, encoding=encoding)
xarray/backends/api.py:752: in to_zarr
    dataset.dump_to_store(store, sync=True, encoding=encoding)
xarray/core/dataset.py:1064: in dump_to_store
    unlimited_dims=unlimited_dims)
xarray/backends/zarr.py:377: in store
    *args, **kwargs)
xarray/backends/common.py:280: in store
    unlimited_dims=unlimited_dims)
xarray/backends/common.py:317: in set_variables
    name, v, check, unlimited_dims=unlimited_dims)
xarray/backends/zarr.py:361: in prepare_variable
    variable, raise_on_invalid=check_encoding)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
variable = <xarray.Variable (y: 2)>
array([0., 1.]), raise_on_invalid = True
    def _extract_zarr_variable_encoding(variable, raise_on_invalid=False):
        encoding = variable.encoding.copy()
    
        valid_encodings = set(['chunks', 'compressor', 'filters',
                               'cache_metadata'])
    
        if raise_on_invalid:
            invalid = [k for k in encoding if k not in valid_encodings]
            if invalid:
                raise ValueError('unexpected encoding parameters for zarr '
>                                'backend:  %r' % invalid)
E               ValueError: unexpected encoding parameters for zarr backend:  ['_FillValue']

[jhamman]

@rabernat - see comment here: https://github.com/pydata/xarray/pull/1793/files#r171290124

[rabernat]

Thanks @jhamman. Ok, not related to #1954