Merge pull request #221 from btolfa/forgotten-virtual-dtor

Added forgotten virtual dtor for `Json::CharReader::Factory`.

(Without this, the destructor of the derived `CharReaderBuilder` would not be called, which is a small memory leak.)

include/json/reader.h

@@ -270,6 +270,7 @@ public:
 
   class Factory {
   public:
+    virtual ~Factory() {}
     /** \brief Allocate a CharReader via operator new().
      * \throw std::exception if something goes wrong (e.g. invalid settings)
      */