bump ember-cli to latest 2.x, this allows use with node 8

Author: ghedamat

package.json

@@ -18,7 +18,7 @@
   "devDependencies": {
     "chai": "^3.5.0",
     "chai-as-promised": "^6.0.0",
-    "ember-cli": "^2.12.0",
+    "ember-cli": "^2.18.2",
     "ember-cli-release": "1.0.0-beta.2",
     "eslint": "^3.18.0",
     "github": "^6.1.0",