Install steps - Add numpy requirement in manual installation steps

Manual installation steps were missing the `requirements/edx/pre.txt` file. So when I followed instructions from the README, without using `scripts/create-dev-env.sh`, I ran `pip install -r requirements/edx/base.txt` directly, which used my system-wide numpy. It had a different version, which caused API versions mistmatch later on when I ran `make cms`, as `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.

Hope this make sense : )