Mark YogaKit Podspec as Deprecated (#1306)

Summary:
Pull Request resolved: https://github.com/facebook/yoga/pull/1306

We are deprecating YogaKit as part of Yoga 2.0, with that version planned as the last release. Mark the podspec as deprecated.

We are also deprecating the Android ViewGroup in a similar way but from the googling I did there is not an equivalent way to do that for Maven artifacts.

Reviewed By: cortinico

Differential Revision: D46663201

fbshipit-source-id: 14e89df8f20470996b5c4b25f098d27cd3f49c7f

YogaKit.podspec

@@ -9,6 +9,7 @@ podspec = Pod::Spec.new do |spec|
   spec.license =  { :type => 'MIT', :file => "LICENSE" }
   spec.homepage = 'https://facebook.github.io/yoga/'
   spec.documentation_url = 'https://facebook.github.io/yoga/docs/'
+  spec.deprecated = true
 
   spec.summary = 'Yoga is a cross-platform layout engine which implements Flexbox.'
   spec.description = 'Yoga is a cross-platform layout engine enabling maximum collaboration within your team by implementing an API many designers are familiar with, and opening it up to developers across different platforms.'