Update rc-dropdown requirement from ~2.0.1 to ~2.4.1

Updates the requirements on [rc-dropdown](https://github.com/react-component/dropdown) to permit the latest version.
- [Release notes](https://github.com/react-component/dropdown/releases)
- [Changelog](https://github.com/react-component/dropdown/blob/master/HISTORY.md)
- [Commits](https://github.com/react-component/dropdown/commits/2.4.1)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot-preview[bot]

package.json

@@ -73,7 +73,7 @@
     "np": "^5.1.0",
     "prettier": "^1.11.1",
     "rc-animate": "^2.10.1",
-    "rc-dropdown": "~2.0.1",
+    "rc-dropdown": "~2.4.1",
     "rc-menu": "^7.4.23",
     "react": "^16.0.0",
     "react-dnd": "^2.5.4",