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)
