chore(package): update uglifyjs-webpack-plugin to version 2.1.3

Closes #141

Author: greenkeeper[bot]

package.json

@@ -43,7 +43,7 @@
     "standard": "^10.0.2",
     "tape": "^4.7.0",
     "uglify-js": "^3.0.20",
-    "uglifyjs-webpack-plugin": "^1.1.2",
+    "uglifyjs-webpack-plugin": "^2.1.3",
     "webpack": "^3.0.0"
   },
   "dependencies": {