GUI: use a local installation of gulp

Change-Id: I5ef201aaf157e630610492e92d910c582ade6513
diff --git a/tools/gui/package.json b/tools/gui/package.json
index 2596add..4c2aeb0 100644
--- a/tools/gui/package.json
+++ b/tools/gui/package.json
@@ -5,7 +5,7 @@
   "main": "index.js",
   "scripts": {
     "test": "echo \"Error: no test specified\" && exit 1",
-    "build": "gulp build"
+    "build": "./node_modules/gulp/bin/gulp.js build"
   },
   "author": "",
   "license": "ISC",