Bump sequelize from 5.8.5 to 5.15.1

Bumps [sequelize](https://github.com/sequelize/sequelize) from 5.8.5 to 5.15.1.
- [Release notes](https://github.com/sequelize/sequelize/releases)
- [Commits](sequelize/sequelize@v5.8.5...v5.15.1)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

package-lock.json

@@ -55,7 +55,7 @@
     "risejs": "^1.4.1",
     "routing-controllers": "=0.7.7",
     "semver": "=5.5.0",
-    "sequelize": "=4.38.0",
+    "sequelize": "=5.15.1",
     "sequelize-typescript": "=0.6.5",
     "shuffle-array": "^1.0.1",
     "socket.io": "=2.1.1",