Bump sequelize from 5.8.7 to 5.15.1

Bumps [sequelize](https://github.com/sequelize/sequelize) from 5.8.7 to 5.15.1.
- [Release notes](https://github.com/sequelize/sequelize/releases)
- [Commits](https://github.com/sequelize/sequelize/compare/v5.8.7...v5.15.1)

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

package.json

@@ -51,7 +51,7 @@
     "pg": "^7.4.3",
     "playmusic": "^2.3.0",
     "sass-loader": "^7.0.1",
-    "sequelize": "^5.3.0",
+    "sequelize": "^5.15.1",
     "spotify-web-api-node": "^4.0.0",
     "superagent": "^5.0.6",
     "vue": "^2.5.17",