Add deployment target to podspec.

Summary: Without specifying the deployment target, it was default to iOS 4.3. This would cause a build error because we are `unable to synthesize weak properties` which didn't come around until iOS 5. Setting default target to iOS 8.

Reviewed By: emilsjolander

Differential Revision: D4481772

fbshipit-source-id: d3be88d0f73533c6242fc1d9045c70ad4628fd21

YogaKit/YogaKit.podspec

@@ -21,6 +21,8 @@ Pod::Spec.new do |spec|
     :tag => 'v2017.01.27.00'
   }
   spec.platform = :ios
+  spec.ios.deployment_target = '8.0'
+  spec.ios.frameworks = 'UIKit'
 
   spec.dependency 'Yoga', '~> 1.0'
   spec.source_files = 'YogaKit/Source/*.{h,m}',