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