chore(package): update ember-cli to version 3.15.1

Closes #180

Author: greenkeeper[bot]

Diff for: package.json

@@ -23,7 +23,7 @@
   "devDependencies": {
     "broccoli-asset-rev": "^2.4.5",
     "ember-ajax": "^3.0.0",
-    "ember-cli": "~2.17.0",
+    "ember-cli": "~3.15.1",
     "ember-cli-dependency-checker": "^2.0.0",
     "ember-cli-eslint": "^4.2.1",
     "ember-cli-htmlbars": "^2.0.1",