CLN: used np.__version__ and removed instantiation
@rkern and others [recommend](http://stackoverflow.com/questions/1520234/how-to-check-which-version-of-numpy-im-using) using `np.__version__` instead of `np.version`. However I realize this is pandas "internals" so there may be some reason for using `np.version` over `np.__version__`. I could not find any such reason. I also removed  a duplicate `LooseVersion(_np_version)` and reused the existing `_nlv` instance.

 - [x] doesn't close any issue, that I'm aware of
 - [x] I ran `nosetests .\pandas\tests\test_compat.py` and 5 tests passed
 - [x] couldn't get the `git` command to work, but `flake8 .\compat\numpy\__init__.py` passes fine
 - [x] whatsnew entry should be covered by `several new features, enhancements, and performance improvements` in the v0.20.0.txt file

## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14988?src=pr) is 84.66% (diff: 100%)
> Merging [#14988](https://codecov.io/gh/pandas-dev/pandas/pull/14988?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will not change coverage

```diff
@@             master     #14988   diff @@
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  Files           144        144          
  Lines         51056      51056          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
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  Hits          43225      43225          
  Misses         7831       7831          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [aba7d25...f56a7ff](https://codecov.io/gh/pandas-dev/pandas/compare/aba7d255a165bbc221be106839c59a876655c0c2...f56a7ff4099ea4acfec7c3cb61b7bb11dd0ffe1d?src=pr) @john-a-m that's a really old link, but in any event, ``__version__`` is usually the recommended way. so thank you.