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
