Bump Laravel Elixir version

Laravel elixir is now at version 4

package.json

@@ -4,7 +4,7 @@
     "gulp": "^3.8.8"
   },
   "dependencies": {
-    "laravel-elixir": "^3.0.0",
+    "laravel-elixir": "^4.0.0",
     "bootstrap-sass": "^3.0.0"
   }
 }