Update mac setup to address error -bash: mkvirtualenv: command not found

[dekuhn]

On new setup getting error -bash: mkvirtualenv: command not found when trying to make the virtual env.

Update developer setup guide for mac with source command fix help based this article Update: https://forsetisecurity.org/docs/latest/develop/dev/setup.html#creating-a-virtualenv

1. Add note to ensure installing with sudo as pip also suggests you can use --user
   sudo pip install virtualenvwrapper

2. Add the below source command otherwise getting error: -bash: mkvirtualenv: command not found when trying to make the virtual env.
   source /usr/local/bin/virtualenvwrapper.sh

Source: https://stackoverflow.com/questions/12232421/virtualenvwrapper-commands-arent-working/12232902