Update react-gemini-scrollbar to include React 16.8 compatibility

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

package.json

@@ -75,7 +75,7 @@
     "flux": "2.1.1",
     "focus-trap-react": "^3.0.5",
     "fuse.js": "^2.2.0",
-    "gemini-scrollbar": "github:matrix-org/gemini-scrollbar#b302279",
+    "gemini-scrollbar": "github:matrix-org/gemini-scrollbar#91e1e566",
     "gfm.css": "^1.1.1",
     "glob": "^5.0.14",
     "highlight.js": "^9.15.8",
@@ -96,7 +96,7 @@
     "react-addons-css-transition-group": "15.3.2",
     "react-beautiful-dnd": "^4.0.1",
     "react-dom": "^15.6.0",
-    "react-gemini-scrollbar": "github:matrix-org/react-gemini-scrollbar#5e97aef",
+    "react-gemini-scrollbar": "github:matrix-org/react-gemini-scrollbar#f644523",
     "resize-observer-polyfill": "^1.5.0",
     "sanitize-html": "^1.18.4",
     "slate": "^0.41.2",