Update CocoaPod badges to be dynamic. (#2886)

This updates the values automatically, which is perfect to reflect the growth of ASDK. I've wanted to do this for ages but the shield.io API didn't have the support up till now. We lose some granularity but I think this is worth it.

Author: aaronschubert0

README.md

@@ -1,7 +1,7 @@
 ![AsyncDisplayKit](https://github.com/AsyncDisplayKit/Documentation/raw/master/docs/static/images/logo.png)
 
-[![Apps Using](https://img.shields.io/badge/Apps%20Using%20ASDK-%3E4,974-28B9FE.svg)](http://cocoapods.org/pods/AsyncDisplayKit)
-[![Downloads](https://img.shields.io/badge/Total%20Downloads-%3E512,000-28B9FE.svg)](http://cocoapods.org/pods/AsyncDisplayKit)
+[![Apps Using](https://img.shields.io/cocoapods/at/AsyncDisplayKit.svg?label=Apps%20Using%20ASDK&colorB=28B9FE)](http://cocoapods.org/pods/AsyncDisplayKit)
+[![Downloads](https://img.shields.io/cocoapods/dt/AsyncDisplayKit.svg?label=Total%20Downloads&colorB=28B9FE)](http://cocoapods.org/pods/AsyncDisplayKit)
 
 [![Platform](https://img.shields.io/badge/platforms-iOS%20%7C%20tvOS-orange.svg)](http://AsyncDisplayKit.org)
 [![Languages](https://img.shields.io/badge/languages-ObjC%20%7C%20Swift-orange.svg)](http://AsyncDisplayKit.org)