fixed issue breaking node v4 compatibility

lib/index.js

@@ -355,10 +355,10 @@ function observe(valueObject, interval, callback) {
   const result = setInterval(() => {
     get(valueObject).then(data => {
       if (JSON.stringify(_data) !== JSON.stringify(data)) {
-        _data = { ...data };
+        _data = Object.assign({}, data );
         callback(data);
       }
-    })
+    });
   }, interval);
   return result;
 }