[PodTarget][Migrator] Store private headers in Pod/Headers/Private instead of Build/

Just in case SCM decides to ignore build directories

lib/cocoapods/installer/migrator.rb

@@ -13,6 +13,7 @@ module Pod
         def migrate(sandbox)
           if sandbox.manifest
             migrate_to_0_34(sandbox) if installation_minor?('0.34', sandbox)
+            migrate_to_0_36(sandbox) if installation_minor?('0.36', sandbox)
           end
         end
 
@@ -46,6 +47,14 @@ module Pod
           delete(Pathname(File.join(ENV['HOME'], 'Library/Caches/CocoaPods/Git')))
         end
 
+        # Migrates from CocoaPods versions prior to 0.36.
+        #
+        def migrate_to_0_36(sandbox)
+          UI.message('Migrating to CocoaPods 0.36') do
+            move(sandbox.root + 'Headers/Build', sandbox.root + 'Headers/Private')
+          end
+        end
+
         # @!group Private helpers
 
         def installation_minor?(target_version, sandbox)

lib/cocoapods/target/pod_target.rb

@@ -19,7 +19,7 @@ module Pod
       @specs = specs
       @target_definition = target_definition
       @sandbox = sandbox
-      @build_headers  = Sandbox::HeadersStore.new(sandbox, 'Build')
+      @build_headers  = Sandbox::HeadersStore.new(sandbox, 'Private')
       @file_accessors = []
     end