chore(package): update ember-cli to version 3.15.0

Closes #45

Author: greenkeeper[bot]

package.json

@@ -37,7 +37,7 @@
     "@ember/optional-features": "^0.7.0",
     "broccoli-asset-rev": "^3.0.0",
     "coveralls": "^3.0.0",
-    "ember-cli": "~3.10.0",
+    "ember-cli": "~3.15.0",
     "ember-cli-code-coverage": "^0.4.2",
     "ember-cli-dependency-checker": "^3.1.0",
     "ember-cli-htmlbars": "^3.0.1",