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

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/