Allow Json::Value to be used in a boolean context (#695)

Must bump soversion too.
diff --git a/include/json/value.h b/include/json/value.h
index be40c30..67d0333 100644
--- a/include/json/value.h
+++ b/include/json/value.h
@@ -400,8 +400,8 @@
   /// otherwise, false.
   bool empty() const;
 
-  /// Return isNull()
-  bool operator!() const;
+  /// Return !isNull()
+  explicit operator bool() const;
 
   /// Remove all object members and array elements.
   /// \pre type() is arrayValue, objectValue, or nullValue