tough-cookie 3.0 breaks node 6

[christianAckman]

https://github.com/salesforce/tough-cookie/blob/master/package.json

"engines": {
    "node": ">=6"
}

https://github.com/salesforce/tough-cookie/blob/v2.5.0/package.json

"engines": {
    "node": ">=0.8"
}