numpy 1.11 support for apply_ufunc

[fujiisoup]

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.

[shoyer]

I think we should just add an better error message about a newer version of NumPy being required. Backporting the changes to np.vectorize would also be pretty easy (see numpy/numpy#8054).