message: bump react-gemini-scrollbar
diff: "react": "^15.4.0",
  "react-addons-css-transition-group": "15.3.2",
  "react-dom": "^15.4.0",
  - "react-gemini-scrollbar": "matrix-org/react-gemini-scrollbar#5e97aef",
  + "react-gemini-scrollbar": "matrix-org/react-gemini-scrollbar#39d858c",
  "sanitize-html": "^1.11.1",
  "text-encoding-utf-8": "^1.0.1",
  "velocity-vector": "vector-im/velocity#059e3b2",
  