COMPAT: use mpl area legend if available
- [x] closes #9161, closes #13544
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Do not use area legend workaround if mpl >= 1.5.0 is available. Legends are now displayed properly if mpl >= 1.5.0.
#9161:

```
df = pd.DataFrame(np.random.rand(10, 3))
df.plot(kind='area', subplots=True, sharex=True, legend=True)
```

![index](https://cloud.githubusercontent.com/assets/1696302/16897682/4ee78324-4bf3-11e6-8113-65d68ec28b09.png)
#13544:

```
df = pd.DataFrame(np.random.rand(20, 5), columns=['A', 'B', 'C', 'D', 'E'])
df.plot(kind='area', linewidth=0.1).legend(loc="center left", bbox_to_anchor=(1.02, 0.5))
```

![index2](https://cloud.githubusercontent.com/assets/1696302/16897684/61eb4370-4bf3-11e6-9472-cec27297b7d1.png)

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13680?src=pr) is 85.30% (diff: 72.72%)

> Merging [#13680](https://codecov.io/gh/pydata/pandas/pull/13680?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #13680   diff @@
==========================================
  Files           139        139          
  Lines         50143      50146     +3   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          42776      42776          
- Misses         7367       7370     +3   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [142c796...968aecc](https://codecov.io/gh/pydata/pandas/compare/142c7968a1918fd56b7ad40ea7c013a48b0c25fc...968aecc1dfaf3e67f9dce8db685cae98c984b42f?src=pr)
 Looks good to me. @TomAugspurger Can you have a look?
 Just the one question, but not really relevant to this PR. +1

Oh, might want a whatsnew entry.
 @TomAugspurger status of this?
