Don't round in set_volume, do round in volume_up/down

`volume_up` and `volume_down` were actually broken because of all the rounding. The chromecast seems to pretty much always return an extremely precise float approximation of whatever you put in, and as such, `volume_up` and `volume_down` need to do this to get consistent volume (inc|dec)rements. Also, given this information, didn't figure there was much point in rounding in `set_volume` at all, since you can do more precise volumes if you want.