Updated Ubuntu install instructions

As mentioned on the Soton mailing list, plus explaining how to get round issues if they already installed `coverage` or `pytest` via packages and then had dependency issues with the original `pip` install.

I should change the RedHat ones as well but would really want to test it on RedHat before doing so. Note that I also switched from a `sudo su -` method to a sudo-per-command which seems better practice than me. (They could get confused with the original instructions because there is never an exit instruction from the root shell.)