Bump sockjs from 0.3.18 to 0.3.21
Bump sockjs from 0.3.18 to 0.3.21

Bumps [sockjs](https://github.com/sockjs/sockjs-node) from 0.3.18 to 0.3.21.
- [Release notes](https://github.com/sockjs/sockjs-node/releases)
- [Changelog](https://github.com/sockjs/sockjs-node/blob/v0.3.21/Changelog)
- [Commits](https://github.com/sockjs/sockjs-node/compare/v0.3.18...v0.3.21)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>
plasma-umass/doppio,plasma-umass/doppio,plasma-umass/doppio