enable microgateway works with  node 8.x  and /node 10.x

node 4.x is out of service
node 6.x will be end of service next year
Please assess if microgateway will work with node 8.x and node 10.x?
Thanks

current engine in package.json
```
 "engines": {
    "node": ">=4.0.0"
}
```

should change to node >=6.0.0