Update package.json build script to newer uglifyjs (#4252 #4246)

Author: jgonggrijp

package.json

@@ -28,7 +28,7 @@
   },
   "scripts": {
     "test": "karma start && coffee test/model.coffee && npm run lint",
-    "build": "uglifyjs backbone.js --mangle --source-map backbone-min.map -o backbone-min.js",
+    "build": "uglifyjs backbone.js --mangle --source-map url=backbone-min.js.map -o backbone-min.js",
     "doc": "docco backbone.js && docco examples/todos/todos.js examples/backbone.localStorage.js",
     "lint": "eslint backbone.js test/*.js"
   },