Update project immutable.js to 2.5.1

libgrabber updated project immutable.js to 2.5.1.

@facebook

### Metadata information

```json
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      "basePath": "dist/",
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      "exclude": []
    },
    "path": "/home/jsdelivr/files/immutable.js",
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    "branchVersions": []
  },
  "version": "2.5.1",
  "updated": true,
  "updatePath": "/home/jsdelivr/files/immutable.js/2.5.1",
  "branch": "immutable.js/2.5.1"
}
```