chore(package): update @ember/optional-features to version 1.2.0

Closes #52

Author: greenkeeper[bot]

package.json

@@ -34,7 +34,7 @@
     "ember-i18n": "^5.3.1"
   },
   "devDependencies": {
-    "@ember/optional-features": "^0.7.0",
+    "@ember/optional-features": "^1.2.0",
     "broccoli-asset-rev": "^3.0.0",
     "coveralls": "^3.0.0",
     "ember-cli": "~3.10.0",