change:track called twice

[Glench]

Related to #22, it seems like when tracks are changed, the change:track event can be fired twice, once when the track changes and once when the track is loaded and begins playing.

To test this, you can do:

GPM.on('change:track', () => {
    process.stdout.write('change:track being called!\n')
})

and play around with skipping to the ends of songs, pressing the next button, etc. The event gets fired more frequently than I'd expect.

I found this when looking into this bug: MarshallOfSound/Google-Play-Music-Desktop-Player-UNOFFICIAL-#3383