fix crash for "C++ Builder" IDE

http://blog.csdn.net/gzliudan/article/details/45264201

Author: Dan Liu

include/json/value.h

@@ -211,7 +211,7 @@ class JSON_API Value {
     void swap(CZString& other);
 
     struct StringStorage {
-      DuplicationPolicy policy_: 2;
+      unsigned policy_: 2;
       unsigned length_: 30; // 1GB max
     };