fix(scripts): adapt validate-engines to respect devDeps
fix(scripts): adapt validate-engines to respect devDeps

it is not only possible to have node as a devDependency, bu also npm!
this means once these two are stated as devDep, we do not need to check
them at all.

```
$ node -v
v4.4.7
$ npm -v
3.9.6
$ npm run versions
npm 5.6.0
node v8.9.4
```

dcos/dcos-ui,dcos/dcos-ui,dcos/dcos-ui,dcos/dcos-ui,dcos/dcos-ui