[Pod] Preserve header directory structure

The React code has relative imports like `#import "../React/Base/Layout.h"`. CocoaPods by default flattens the header files so they're all in a single directory, but specifying `header_mappings_dir` in the podspec preserves the directory structure.

React.podspec

@@ -24,6 +24,7 @@ Pod::Spec.new do |s|
   s.platform           = :ios, "7.0"
   s.prepare_command    = 'npm install'
   s.preserve_paths     = "cli.js", "Libraries/**/*.js", "lint", "linter.js", "node_modules", "package.json", "packager", "PATENTS", "react-native-cli"
+  s.header_mappings_dir = "."
 
   s.subspec 'Core' do |ss|
     ss.source_files    = "React/**/*.{c,h,m}"