Update package.json

Author: petecooper

package.json

@@ -10,7 +10,7 @@
     "url": "https://github.com/textpattern/textpattern-plugins-website/issues"
   },
   "devDependencies": {
-    "grunt": "1.4.1",
+    "grunt": "1.5.3",
     "grunt-cli": "1.4.3",
     "grunt-concurrent": "3.0.0",
     "grunt-contrib-clean": "2.0.0",