Nuget crawls for newer packages failing due to api version

For example:
https://github.com/clearlydefined/crawler/blob/master/providers/fetch/nugetFetch.js#L70
```
`${baseUrl}/v3/registration4-gz-semver2/${spec.name.toLowerCase()}/${spec.revision}.json`
```
This doesn't work with new packages, needs `registration5`
Not sure about the other routes.

Maybe code should call `https://api.nuget.org/v3/index.json` at startup to get the current routes.