CI: Windows build failing
e.g. https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=1680&view=logs

seems to be affecting multiple builds.

```
2018-10-24T15:55:48.9463601Z ##[command]"C:\Windows\system32\cmd.exe" /D /E:ON /V:OFF /S /C "CALL "D:\a\_temp\ecbf7c44-999c-40cd-8868-eadec34d4bc7.cmd""
2018-10-24T15:56:04.8632197Z ImportError while loading conftest 'D:\a\1\s\pandas\conftest.py'.
2018-10-24T15:56:05.2570958Z pandas\__init__.py:42: in <module>
2018-10-24T15:56:05.2571607Z     from pandas.core.api import *
2018-10-24T15:56:05.2571761Z pandas\core\api.py:10: in <module>
2018-10-24T15:56:05.2571931Z     from pandas.core.groupby import Grouper
2018-10-24T15:56:05.2572072Z pandas\core\groupby\__init__.py:1: in <module>
2018-10-24T15:56:05.2572252Z     from pandas.core.groupby.groupby import GroupBy  # flake8: noqa
2018-10-24T15:56:05.2572423Z pandas\core\groupby\groupby.py:40: in <module>
2018-10-24T15:56:05.2572592Z     from pandas.core.frame import DataFrame
2018-10-24T15:56:05.2572776Z pandas\core\frame.py:98: in <module>
2018-10-24T15:56:05.2572941Z     from pandas.core.series import Series
2018-10-24T15:56:05.2573946Z pandas\core\series.py:18: in <module>
2018-10-24T15:56:05.2574292Z     import pandas.plotting._core as gfx
2018-10-24T15:56:05.2574542Z pandas\plotting\__init__.py:15: in <module>
2018-10-24T15:56:05.2574711Z     from pandas.plotting._converter import (
2018-10-24T15:56:05.2575076Z pandas\plotting\_converter.py:8: in <module>
2018-10-24T15:56:05.2575492Z     import matplotlib.units as units
2018-10-24T15:56:05.2576398Z C:\Miniconda\envs\pandas\lib\site-packages\matplotlib\__init__.py:1111: in <module>
2018-10-24T15:56:05.2576496Z     rcParamsOrig = RcParams(rcParams.copy())
2018-10-24T15:56:05.2577252Z C:\Miniconda\envs\pandas\lib\site-packages\matplotlib\__init__.py:891: in __getitem__
2018-10-24T15:56:05.2577679Z     from matplotlib import pyplot as plt
2018-10-24T15:56:05.2578184Z C:\Miniconda\envs\pandas\lib\site-packages\matplotlib\pyplot.py:32: in <module>
2018-10-24T15:56:05.2578480Z     import matplotlib.colorbar
2018-10-24T15:56:05.2579211Z C:\Miniconda\envs\pandas\lib\site-packages\matplotlib\colorbar.py:28: in <module>
2018-10-24T15:56:05.2579427Z     import matplotlib.artist as martist
2018-10-24T15:56:05.2579803Z E   AttributeError: module 'matplotlib' has no attribute 'artist'
2018-10-24T15:56:05.4589056Z ##[error]Cmd.exe exited with code '4'.
2018-10-24T15:56:05.4970962Z ##[section]Finishing: Test
```