--version fails during install

I might be doing this wrong, but when running:

```
helm plugin install --version 0.18.1 https://github.com/instrumenta/helm-conftest
```

I get:
```
Error: requested version "0.18.1" does not exist for plugin "https://github.com/instrumenta/helm-conftest"
```

Should I be able to install a specific version/otherwise pin the version?