Always include numpy when running conda skeleton pypi
Always include numpy when running conda skeleton pypi

A significant number of packages use numpy.distutils, meaning they import
numpy in setup.py, so it's worth just doing this always, instead of requiring
to conda install -n _build numpy every time.

Fixes #824.

mwcraig/conda-build,shastings517/conda-build,shastings517/conda-build,dan-blanchard/conda-build,ilastik/conda-build,takluyver/conda-build,takluyver/conda-build,takluyver/conda-build,frol/conda-build,mwcraig/conda-build,ilastik/conda-build,sandhujasmine/conda-build,frol/conda-build,frol/conda-build,rmcgibbo/conda-build,ilastik/conda-build,dan-blanchard/conda-build,shastings517/conda-build,rmcgibbo/conda-build,rmcgibbo/conda-build,sandhujasmine/conda-build,mwcraig/conda-build,sandhujasmine/conda-build,dan-blanchard/conda-build