`source` command on available on all Linuxes

There is a gotcha with shell `source` command with `virtualenv`. `source` is not standard and not available on all Linuxes (or more accurately on their default shells). Instead, more common `.`. If you do `source` then some people might simply get `Command not found` when they try to follow the installation instructions.

Namely the late Ubuntus and its `dash` shell might be problematic. The issue is discussed here: https://wiki.ubuntu.com/DashAsBinSh#source