Update Workbox dependency (#5527)

Author: jeffposnick

packages/react-scripts/package.json

@@ -67,7 +67,7 @@
     "webpack": "4.19.1",
     "webpack-dev-server": "3.1.9",
     "webpack-manifest-plugin": "2.0.4",
-    "workbox-webpack-plugin": "3.6.2"
+    "workbox-webpack-plugin": "3.6.3"
   },
   "devDependencies": {
     "react": "^16.3.2",