Missing Bus Device requirement in setup.py

[tannewt]

https://forums.adafruit.com/viewtopic.php?f=60&t=169030

We need to add Bus Device as a requirement so it is installed by pip as a dependency.

[ladyada]

@dherrada wanna try submitting a PR?

[evaherrada]

@ladyada Sure. I'd love to give it a go

[evaherrada]

Fixed in #2