Update project immutable.js to 2.6.0

libgrabber updated project immutable.js to 2.6.0.

@facebook

### Metadata information

```json
{
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    "packageManager": "github",
    "name": "immutable.js",
    "repo": "facebook/immutable-js",
    "files": {
      "basePath": "dist/",
      "include": ["Immutable.min.js", "Immutable.d.ts"],
      "exclude": []
    },
    "path": "/home/jsdelivr/files/immutable.js",
    "localVersions": ["2.0.17", "2.1.0", "2.2.0", "2.2.1", "2.2.2", "2.2.3", "2.3.0", "2.3.1", "2.3.2", "2.4.0", "2.5.0", "2.5.1"],
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    "branchVersions": []
  },
  "version": "2.6.0",
  "updated": true,
  "updatePath": "/home/jsdelivr/files/immutable.js/2.6.0",
  "branch": "immutable.js/2.6.0"
}
```