Skip duplicate versions

I test most of my packages on many versions of Julia

```
      matrix:
        version:
          - 'min'
          - 'lts'
          - '1'
          - 'pre'
          - 'nightly'
```

However, sometimes these overlap. Right now, `1` and `pre` are the same and I run tests twice for that version. I'd like to skip one of those.