Fixed the build issue for oss

Summary:
Our OSS build was failing with the latest version of cocoapods(1.6.0). The reason was that the `post_install` script of adding a `SWIFT_VERSION` was not getting applied while building the project. So I added a `swift_version` in the YogaKit pod itself. So with this change we no longer need the `post_install` script to add `SWIFT_VERSION`.

I have also released a new version of YogaKit(1.10.0) on the CocoaPods. Usually the swift pods should specify the swift_version which is being used to develop it.Due to unknown reason, it was never mentioned in the podspec.

Reviewed By: jknoxville

Differential Revision: D13990363

fbshipit-source-id: 3c2f814765c03683fbedd17b3607ff0e6cdef566

YogaKit.podspec

@@ -6,7 +6,7 @@
 #
 podspec = Pod::Spec.new do |spec|
   spec.name = 'YogaKit'
-  spec.version = '1.9.0'
+  spec.version = '1.10.0'
   spec.license =  { :type => 'MIT', :file => "LICENSE" }
   spec.homepage = 'https://facebook.github.io/yoga/'
   spec.documentation_url = 'https://facebook.github.io/yoga/docs/api/yogakit/'
@@ -28,6 +28,7 @@ podspec = Pod::Spec.new do |spec|
   spec.source_files = 'YogaKit/Source/*.{h,m,swift}'
   spec.public_header_files = 'YogaKit/Source/{YGLayout,UIView+Yoga}.h'
   spec.private_header_files = 'YogaKit/Source/YGLayout+Private.h'
+  spec.swift_version = '3.0'
 end
 
 # See https://github.com/facebook/yoga/pull/366