hack for matplotlib's pip install problems - numpy dep

workaround for matplotlib and asteval install bug: - numpy must already be installed
see http://stackoverflow.com/questions/11797688/matplotlib-requirements-with-pip-install-in-virtualenv
https://github.com/matplotlib/matplotlib/wiki/MEP11

numpy must be already installed, and requirements.txt file does not work, so do it manually
in build.sh before calling requirements.txt file

Conflicts:
	build.sh
	external/common/requirements.txt