fix: add missing dependency

Author: gajus

demo/package.json

@@ -7,6 +7,7 @@
     "webpack": "^2.2.0-rc.3"
   },
   "devDependencies": {
+    "babel-core": "^6.21.0",
     "babel-loader": "^6.2.10",
     "babel-plugin-transform-react-jsx": "^6.8.0",
     "css-loader": "^0.26.1",