Unhandled warning from NumPy dev in median
```python
In [1]: import pandas as pd

In [2]: import numpy as np

In [3]: x = np.random.randn(30, 4)

In [4]: x[5:10] = np.nan

In [5]: pd.DataFrame(x).median(1).head()
/Users/taugspurger/miniconda3/envs/travis-37-numpydev/lib/python3.7/site-packages/numpy/lib/function_base.py:3267: RuntimeWarning: All-NaN slice encountered
  r = func(a, **kwargs)
Out[5]:
0   -0.602415
1   -0.118699
2    0.744368
3   -0.993376
4   -0.304693
dtype: float64

In [6]: np.__version__
Out[6]: '1.16.0.dev0+88cbd3d'
```

Do we want to silence these? 

Working around this in https://github.com/pandas-dev/pandas/pull/22699