TST: matplotlib 2.0 fix in log limits for barplot (GH14808)
Closes #14808
👍  when the tests pass ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14957?src=pr) is 84.66% (diff: 100%)
> Merging [#14957](https://codecov.io/gh/pandas-dev/pandas/pull/14957?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

```diff
@@             master     #14957   diff @@
==========================================
  Files           144        144          
  Lines         51030      51047    +17   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43198      43217    +19   
+ Misses         7832       7830     -2   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [8b497e4...1f1a80d](https://codecov.io/gh/pandas-dev/pandas/compare/8b497e49387ad167acb1c8ae31d67676e30add5e...1f1a80d61cc1516356e72e950045ca0d55408307?src=pr) @TomAugspurger had to change it a bit further (the build in allowed failures for matplotlib 2.0cr2 was stll failing). Merged now, but I am wondering if we should actually test those 'implementation' details of matplotlib, and not just that the plot works. I agree that the test isn't all that useful.