depend on uglifyjs-webpack-plugin

uglifyjs-webpack-plugin is used in webpack.config.js.

package.json

@@ -79,6 +79,7 @@
     "stats.js": "^0.17.0",
     "tap": "^12.0.1",
     "tiny-worker": "^2.1.1",
+    "uglifyjs-webpack-plugin": "1.2.7",
     "webpack": "^4.16.5",
     "webpack-cli": "^3.1.0",
     "webpack-dev-server": "^3.1.5"