update Grunt dev dependency

package.json

@@ -25,7 +25,7 @@
   },
   "homepage": "https://github.com/owncloud/mail",
   "devDependencies": {
-    "grunt": "^0.4.5",
+    "grunt": "^1.0.1",
     "grunt-cli": "^0.1.13",
     "grunt-contrib-jshint": "^0.11.3",
     "grunt-jscs": "^1.8.0",