Updated Example/Podfile to explicitly specify Artsy+UILabels.podspec

@orta 

I was having difficulty getting this setup on my machine.
I'm not sure if it's something I was doing wrong or if my environment doesn't match yours.

I was getting this output when I tried running `pod install` from Example folder.
![screen shot 2014-11-06 at 23 09 48](https://cloud.githubusercontent.com/assets/1572886/4945723/ad59606a-660a-11e4-9182-1fbeff7d76e2.png)

Managed to get around it by explicitly specifying `Artsy+UILabels.podspec` in the Podfile