level-filesystem@1.2.0 depends on outdated level-sublevel@^5.2.0 (current version is 6.6.1)

`npm install` gives this warning:
```
npm WARN deprecated object-keys@0.2.0: Please update to the latest object-keys
```

`npm ls object-keys` shows it as an indirect dependency of `level-filesystem`:
```
│ ├─┬ level-filesystem@1.2.0
│ │ └─┬ level-sublevel@5.2.3
│ │ └─┬ xtend@2.0.6
│ │ └── object-keys@0.2.0
```

Reading `npm info` shows that `level-filesystem@1.2.0` is the current version, and depends on an outdated version of `level-sublevel`.

Reproduce the warning with npm install and inspect the package metadata that declares level-filesystem's level-sublevel dependency. Verify the dependency tree with npm ls object-keys, update the dependency to a compatible current version, and confirm that the deprecated object-keys warning is gone.

Written by the indexing model from the issue text.