environment: update virtual env setup instructions. Fix #657. (#658)
environment: update virtual env setup instructions. Fix #657. (#658)

The `venv` module seems the preferred way to create virtual environments these days and even though the bug hasn't been resolved completely, testing (on Xenial and Bionic) shows that there do not appear to be any problems with pip any more.
ros-infrastructure/ros_buildfarm,ros-infrastructure/ros_buildfarm,ros-infrastructure/ros_buildfarm