gnu: python-numpy: Update to 1.13.1.

* gnu/packages/python.scm (python-numpy, python2-numpy): Update to 1.13.1.
[source]: Download the zip release from pypi instead of a git snapshot.
[native-inputs]: Add unzip.
[arguments]: Set SHELL environment variable.

Co-authored by Leo Famulari <leo@famulari.name>