fix(package): update sequelize to version 5.21.11

Closes #81

Author: greenkeeper[bot]

package.json

@@ -39,7 +39,7 @@
     "graphql-resolvers": "^0.2.2",
     "jsonwebtoken": "^8.4.0",
     "pg": "^7.6.1",
-    "sequelize": "^4.41.2",
+    "sequelize": "^5.21.11",
     "uuid": "^3.3.2"
   }
 }