CI: mpl test failures
https://travis-ci.org/pandas-dev/pandas/jobs/463295292; keep failing even with a restart.

```
=================================== FAILURES ===================================
_____________________ TestTSPlot.test_is_error_nozeroindex _____________________
[gw0] linux -- Python 3.6.6 /home/travis/miniconda3/envs/pandas-dev/bin/python
self = <pandas.tests.plotting.test_datetimelike.TestTSPlot object at 0x7f049594ef98>
    def test_is_error_nozeroindex(self):
        # GH11858
        i = np.array([1, 2, 3])
        a = DataFrame(i, index=i)
>       _check_plot_works(a.plot, xerr=a)
pandas/tests/plotting/test_datetimelike.py:89: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
pandas/tests/plotting/test_datetimelike.py:1562: in _check_plot_works
    pickle.dump(fig, path)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
m = <bound method AffineBase.transform of <matplotlib.transforms.BlendedAffine2D object at 0x7f0495e0e390>>
    def _pickle_method(m):
        """Handle pickling issues with class instance."""
>       if m.im_self is None:
E       AttributeError: 'function' object has no attribute 'im_self'
../../../miniconda3/envs/pandas-dev/lib/python3.6/site-packages/statsmodels/graphics/functional.py:32: AttributeError
________________________ TestTSPlot.test_timedelta_plot ________________________
[gw0] linux -- Python 3.6.6 /home/travis/miniconda3/envs/pandas-dev/bin/python
self = <pandas.tests.plotting.test_datetimelike.TestTSPlot object at 0x7f04951ab208>
    def test_timedelta_plot(self):
        # test issue #8711
        s = Series(range(5), timedelta_range('1day', periods=5))
        _, ax = self.plt.subplots()
>       _check_plot_works(s.plot, ax=ax)
pandas/tests/plotting/test_datetimelike.py:1450: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
pandas/tests/plotting/test_datetimelike.py:1562: in _check_plot_works
    pickle.dump(fig, path)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
m = <bound method AffineBase.transform of <matplotlib.transforms.BlendedAffine2D object at 0x7f0494f0b160>>
    def _pickle_method(m):
        """Handle pickling issues with class instance."""
>       if m.im_self is None:
E       AttributeError: 'function' object has no attribute 'im_self'
../../../miniconda3/envs/pandas-dev/lib/python3.6/site-packages/statsmodels/graphics/functional.py:32: AttributeError
```