board_revision.py does not return the correct revision on new models

[Makalov]

The revision() function in board_revision.py needs to be updated. This function is not written to handle inputs from newer models. This causes the init function in i2c.py to use the wrong bus as default.

Right now on Pi 3 models the this cause the default bus to be set to 0 rather than 1.

https://www.raspberrypi-spy.co.uk/2012/09/checking-your-raspberry-pi-board-version/