chore(package): update eslint-plugin-ember to version 8.4.0

Closes #31

Author: greenkeeper[bot]

package.json

@@ -42,7 +42,7 @@
     "ember-source": "~3.6.0",
     "ember-source-channel-url": "^1.1.0",
     "ember-try": "^1.0.0",
-    "eslint-plugin-ember": "^5.2.0",
+    "eslint-plugin-ember": "^8.4.0",
     "eslint-plugin-node": "^7.0.1",
     "loader.js": "^4.7.0",
     "qunit-dom": "^0.8.0"