chore(package): update terser-webpack-plugin to version 2.3.3

Closes #581

Author: greenkeeper[bot]

package.json

@@ -177,7 +177,7 @@
     "sass-loader": "^7.0.1",
     "style-loader": "^0.23.0",
     "sw-precache-webpack-plugin": "^0.11.3",
-    "terser-webpack-plugin": "^1.2.3",
+    "terser-webpack-plugin": "^2.3.3",
     "url-loader": "^1.0.1",
     "webpack": "^4.7.0",
     "webpack-cli": "^3.0.0",