Fix control signal

When I called #show it looked like the tape was displaying the previous set of pixels, not the ones I'd just set. e.g.:

```ruby
blinkytape.display_color 255,0,0
sleep 2
blinkytape.display_color 0,255,0
# tape would be red, not green
sleep 2
blinkytape.display_color 0,0,255
# tape would be green, not blue
```

The control signal in the python version is 0 + 0 + 255, instead of just 255 https://github.com/Blinkinlabs/BlinkyTape_Python/blob/master/BlinkyTape.py#L103 using this seems to work