Stop using internal Mopidy APIs

[jodal]

Mopidy-TtsGpio is using the internal function mopidy.utils.process.exit_process() in one place:

• main_menu.py:43

As this method is internal and not supposed to be used by Mopidy extensions, I recommend using other means for stopping Mopidy, similar to your methods for shutting down the device with os.system().

If mopidy/mopidy#1173 is merged, Mopidy-TtsGpio will break with Mopidy 1.1.