Use backingStorePixelRatio instead of devicePixelRatio

[Vinicius Nakamura]

First of all, thanks for publishing this package. It's so much easier to use it as an independent script.

I'm getting some false negatives when running it on a retina MacBook. Then I noticed that the there's an update to the Moderniz code that's not in your package:
Modernizr/Modernizr#2420

More details in the Modernizr issue:
Modernizr/Modernizr#2419