Remove std::swap<Json::Value> in favor of ADL

Comply with http://en.cppreference.com/w/cpp/concept/Swappable
Don't open namespace std.

Author: BillyDonahue

include/json/value.h

@@ -880,14 +880,9 @@ class JSON_API ValueIterator : public ValueIteratorBase {
   pointer operator->() const { return &deref(); }
 };
 
-} // namespace Json
-
+inline void swap(Value& a, Value& b) { a.swap(b); }
 
-namespace std {
-/// Specialize std::swap() for Json::Value.
-template<>
-inline void swap(Json::Value& a, Json::Value& b) { a.swap(b); }
-}
+} // namespace Json
 
 #pragma pack(pop)