egg-Sequelize start with errors about  'database engine version is not supported', but run dev works well
## What happens?
```
[egg-scripts] Got error when startup: 
[egg-scripts] (node:5395) [SEQUELIZE0006] DeprecationWarning: This database engine version is not supported, please update your database server. More information https://github.com/sequelize/sequelize/blob/master/ENGINE.md
```

  "egg-sequelize": "^6.0.0",
    "mysql2": "^2.2.1",

## Mini Showcase Repository(REQUIRED)
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> Use `npm init egg --type=simple bug` then upload to your GitHub

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## How To Reproduce

**Steps to reproduce the behavior:**
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**Expected behavior**
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## Context
- **Node Version**:
- **Egg Version**:
- **Plugin Name**:
- **Plugin Version**:
- **Platform**:
