Bump elixir requirement.

package.json

@@ -2,6 +2,6 @@
   "private": true,
   "devDependencies": {
     "gulp": "^3.8.8",
-    "laravel-elixir": "^1.0.0"
+    "laravel-elixir": "^2.0.0"
   }
 }