Fix setup.py to resolve numpy requirement
during pre-install. This should allow
'pip install numpy cython scipy scikit-learn'
to install scikit.

This fix is based of issue and fixes in Scipy:
https://github.com/scipy/scipy/pull/453
https://github.com/branden/scipy/commit/c0555433a0bfb00146c6e195c8e0adc26bb4dab5