update uglifyjs-webpack-plugin away from beta

Author: ewingrj

packages/react-scripts/package.json

@@ -56,7 +56,7 @@
     "webpack": "3.5.1",
     "webpack-dev-server": "2.7.1",
     "webpack-manifest-plugin": "1.2.1",
-    "uglifyjs-webpack-plugin": "^1.0.0-beta.2",
+    "uglifyjs-webpack-plugin": "^1.2.5",
     "whatwg-fetch": "2.0.3"
   },
   "devDependencies": {