I noticed the failing in rtd
http://xarray.pydata.org/en/stable/computation.html#missing-values
is because it still uses numpy=1.11 which does not support signature argument for np.vectorize.

This can be easily fixed (just bumping up numpy's version on rtd),
but as our minimum requirement is numpy==1.11, we may need to take care of this in xr.apply_ufunc.