Update installation instruction for CP 1.7+ (#522)

Summary:
This PR updates the installation instruction for Cocoapods 1.7+ as the previous installation instruction was broken for the latest Cocoapods version

Solves https://github.com/facebook/flipper/issues/478 and https://github.com/facebook/flipper/issues/495

## Changelog

- Updated the Documentation
- Updated the sample app
Pull Request resolved: https://github.com/facebook/flipper/pull/522

Test Plan:
- Travis CI
- Also tested locally by building sample app.

Reviewed By: jknoxville

Differential Revision: D16902023

Pulled By: priteshrnandgaonkar

fbshipit-source-id: 19cc01c842f93ab3565c302f8258ca88dbdecfb2

iOS/Sample/Podfile

@@ -1,7 +1,6 @@
 project 'Sample.xcodeproj'
 source 'https://github.com/facebook/Sonar.git'
 source 'https://github.com/CocoaPods/Specs'
-swift_version = "4.1"
 
 target 'Sample' do
   platform :ios, '9.0'
@@ -11,8 +10,37 @@ target 'Sample' do
   pod 'FlipperKit/FlipperKitUserDefaultsPlugin', :path => '../../FlipperKit.podspec'
   pod 'FlipperKit/FlipperKitExamplePlugin', :path => '../../FlipperKit.podspec'
   pod 'Flipper', :path => '../../Flipper.podspec'
+
+  # If you use `use_frameworks!` in your Podfile,
+  # uncomment the below $static_framework array and also
+  # the pre_install section.  This will cause Flipper and
+  # it's dependencies to be built as a static library and all other pods to
+  # be dynamic.
+  # $static_framework = ['FlipperKit', 'Flipper', 'Flipper-Folly',
+  #   'CocoaAsyncSocket', 'ComponentKit', 'DoubleConversion',
+  #   'glog', 'Flipper-PeerTalk', 'Flipper-RSocket', 'Yoga', 'YogaKit',
+  #   'CocoaLibEvent', 'openssl-ios-bitcode', 'boost-for-react-native']
+  #
+  # pre_install do |installer|
+  #   Pod::Installer::Xcode::TargetValidator.send(:define_method, :verify_no_static_framework_transitive_dependencies) {}
+  #   installer.pod_targets.each do |pod|
+  #       if $static_framework.include?(pod.name)
+  #         def pod.build_type;
+  #           Pod::Target::BuildType.static_library
+  #         end
+  #       end
+  #     end
+  # end
+
   # It also adds -DFB_SONARKIT_ENABLED=1 flag to OTHER_CFLAGS, necessary to build expose Flipper classes in the header files
   post_install do |installer|
+    installer.pods_project.targets.each do |target|
+      if target.name == 'YogaKit'
+        target.build_configurations.each do |config|
+          config.build_settings['SWIFT_VERSION'] = '4.1'
+        end
+      end
+    end
     file_name = Dir.glob("*.xcodeproj")[0]
     app_project = Xcodeproj::Project.open(file_name)
     app_project.native_targets.each do |target|