remove venv create -n option; add a venv create -g option which means gi...

remove venv create -n option; add a venv create -g option which means give access to the global site-packages dir to the vitrualenv; the previous -n option is now the default behaviour in recent version of virtualenv

`--no-site-packages` option of virtualenv is deprecated and made to the default behaviour. [See here](https://github.com/pypa/virtualenv/blob/develop/virtualenv.py#L835). Meanwhile, a `--system-site-packages` option is added. virtualenv 1.8.4 used by pythonbrew has already made this change. I remove the -n option and add a -g option to create a venv with the `--system-site-packages` option.