Bump eslint in /Single-Page-App-with-REST-API/backend

Bumps [eslint](https://github.com/eslint/eslint) from 2.13.1 to 6.6.0.
- [Release notes](https://github.com/eslint/eslint/releases)
- [Changelog](https://github.com/eslint/eslint/blob/master/CHANGELOG.md)
- [Commits](https://github.com/eslint/eslint/compare/v2.13.1...v6.6.0)

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

Single-Page-App-with-REST-API/backend/package.json

@@ -20,7 +20,7 @@
     "swagmock": "~0.0.2"
   },
   "devDependencies": {
-    "eslint": "^2",
+    "eslint": "^6",
     "istanbul": "~0.4.3",
     "is-my-json-valid": "^2.13.1",
     "js-yaml": "^3.2.6",