Merge pull request #499 from Metaswitch/renovate/gulp-uglify-3.x

Update dependency gulp-uglify to v3

Author: maxdymond

docs/themes/purehugo/package.json

@@ -3,6 +3,6 @@
     "gulp": "^5.0.0",
     "gulp-concat": "^2.5.2",
     "gulp-minify-css": "^1.1.0",
-    "gulp-uglify": "^1.2.0"
+    "gulp-uglify": "^3.0.0"
   }
 }