message: Add core-js to package.json
diff: "check-node-version": "3.2.0",
  "concurrently": "3.5.1",
  "copy-webpack-plugin": "4.5.1",
  + "core-js": "2.5.5",
  "coveralls": "3.0.0",
  "css-loader": "0.28.11",
  "electron": "1.8.4",
  