Unresolved or ambiguous spec for stringio gem

For the last year or so, I've noticed this warning frequently appear after installing standard:

```
Installing standard 1.49.0 (was 1.47.0)
WARN: Unresolved or ambiguous specs during Gem::Specification.reset:
      stringio (>= 0)
      Available/installed versions of this gem:
      - 3.1.6
      - 3.1.5
      - 3.1.1
```

