Bump sequelize from 3.14.2 to 6.3.5 in /todo-backend

Bumps [sequelize](https://github.com/sequelize/sequelize) from 3.14.2 to 6.3.5.
- [Release notes](https://github.com/sequelize/sequelize/releases)
- [Commits](https://github.com/sequelize/sequelize/compare/v3.14.2...v6.3.5)

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

todo-backend/package.json

@@ -10,7 +10,7 @@
   },
   "dependencies": {
     "restify": "4.3.0",
-    "sequelize": "3.14.2",
+    "sequelize": "6.3.5",
     "mysql": "2.9.0"
   }
 }