Install steps - Add numpy requirement in manual installation steps

When `pip install -r requirements/edx/pre.txt` is omitted before running
`pip install -r requirements/edx/base.txt`, it would cause API versions
mistmatch when `make cms` is run. `make cms` would automatically run pip
with pre.txt and install a different version of numpy, *after* base.txt
and post.txt have been run through pip.
Merge pull request #34 from antoviaque/install-requirements-pre

Install steps - Add numpy requirement in manual installation steps