describe() doesn't ignore Nan anymore
`.describe()` is not ignoring the `Nan` values at percentiles, so it gives `Nan`:

http://pandas.pydata.org/pandas-docs/stable/basics.html#summarizing-data-describe

Before 0.18 it was working as expected:

http://pandas.pydata.org/pandas-docs/version/0.17.0/basics.html#summarizing-data-describe
