fix(package): update sequelize to version 5.21.8

Closes #24

Author: greenkeeper[bot]

package.json

@@ -37,7 +37,7 @@
     "mysql2": "^1.6.1",
     "prettier": "^1.14.2",
     "pretty-quick": "^1.6.0",
-    "sequelize": "^4.38.0",
+    "sequelize": "^5.21.8",
     "superagent": "3.8.3",
     "supertest": "3.1.0"
   },