fix logic error

Author: Judahmeek

react_ujs/dist/react_ujs.js

@@ -189,7 +189,7 @@ var ReactRailsUJS = {
 
   unmountRoot: function(node) {
     var targetRoot = this.findRoot(node);
-    if (targetRoot) {
+    if (!targetRoot) {
       return;
     }