Fixed #1429: "pip install numpy scipy" was failing

It was failing because of the pre installation request from pip to ./setup.py egg_info (which is perfectly valid request and shouldn't fail), before numpy actually was installed.

This is pretty popular issue:
http://projects.scipy.org/scipy/ticket/1429
https://github.com/pypa/pip/issues/25
https://github.com/pypa/pip/issues/272
https://github.com/sjsrey/pysal/issues/207
http://bb10.com/python-testing-general/2011-06/msg00028.html
etc...

This really annoying when using tox - with this fix just set up requirements file and be sure that it will work on any machine (including on machines has already preinstalled numpy with different version that required).

Fix is straightforward - not to use numpy for commands that are not connected with build and install.