Build(deps-dev): Bump eslint from 5.16.0 to 6.8.0

Bumps [eslint](https://github.com/eslint/eslint) from 5.16.0 to 6.8.0.
- [Release notes](https://github.com/eslint/eslint/releases)
- [Changelog](https://github.com/eslint/eslint/blob/master/CHANGELOG.md)
- [Commits](eslint/eslint@v5.16.0...v6.8.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

package.json

@@ -64,7 +64,7 @@
     "ember-source": "~3.18.1",
     "ember-source-channel-url": "^1.1.0",
     "ember-try": "^1.3.0",
-    "eslint": "5",
+    "eslint": "6",
     "eslint-config-prettier": "^6.11.0",
     "eslint-plugin-ember": "^7.6.0",
     "eslint-plugin-node": "^9.2.0",