Bump grunt-dev dependency.

For safety, amber dev is >0.18.1, compiled w/ newest amber.

package.json

@@ -25,7 +25,7 @@
     "node": "0.10.x || 0.12.x || >=4.0.0"
   },
   "devDependencies": {
-    "amber-dev": "^0.8.2",
+    "amber-dev": "^0.8.7",
     "grunt": "^0.4.5",
     "grunt-contrib-clean": "^0.6.0",
     "grunt-contrib-requirejs": "^0.4.4",