Add the -H option to sudo as asked by pip

[Flaburgan]

sudo pip install -U virtualenvwrapper pip
The directory '/home/fla/.cache/pip/http' or its parent directory is not owned by the current user and the cache has been disabled. Please check the permissions and owner of that directory. If executing pip with sudo, you may want sudo's -H flag.
The directory '/home/fla/.cache/pip' or its parent directory is not owned by the current user and caching wheels has been disabled. check the permissions and owner of that directory. If executing pip with sudo, you may want sudo's -H flag.

[jaywink]

I have no idea really why you would need the -H flag - ie why is it using your own home folders instead of root.

Anyway, I think I'll just remove the whole part and just say "install Python dependencies into a virtualenv", with a link to some generic tutorial. Not worth trying to provide working commands for all environments here :)

I'll close this PR, thanks for notifying about the problem with the commands.