chore(package): update ember-cli to version 2.8.0

https://greenkeeper.io/

Author: greenkeeperio-bot

package.json

@@ -23,7 +23,7 @@
   "devDependencies": {
     "broccoli-asset-rev": "^2.4.2",
     "ember-ajax": "^2.0.1",
-    "ember-cli": "2.7.0",
+    "ember-cli": "2.8.0",
     "ember-cli-app-version": "^2.0.0",
     "ember-cli-babel": "^5.1.6",
     "ember-cli-dependency-checker": "^1.2.0",