Map lts versions to semver in travis rule

[SamVerschueren]

We now also support the nvm aliases lts/argon (Node.js 4) and lts/boron (Node.js 6). Currently the travis rule does not prevent something like this.

{
    "name": "my-module",
    "engines": {
        "node": ">=6"
    }
}

language: node_js
node_js:
  - lts/boron
  - lts/argon

The rule should fail indicating that lts/argon is not supported. This can be done by mapping these aliases their semantic version equivalent and work from that.