python 3.6 tests break with bottleneck installed

[max-sixty]

Installing 3.6 environment (from the ci path in xarray): tests pass on master
Then installing bottleneck: 3 tests in test_dataarray.py fail on master

I can debug further unless anyone has a view

[jhamman]

@MaximilianR - can you try testing with this branch: #1278

I'm guessing the errors you're getting are related.

[max-sixty]

Fixed