Bump rxjava from 2.0.4 to 2.2.21
Bump rxjava from 2.0.4 to 2.2.21

Bumps [rxjava](https://github.com/ReactiveX/RxJava) from 2.0.4 to 2.2.21.
- [Release notes](https://github.com/ReactiveX/RxJava/releases)
- [Changelog](https://github.com/ReactiveX/RxJava/blob/v2.2.21/CHANGES.md)
- [Commits](https://github.com/ReactiveX/RxJava/compare/v2.0.4...v2.2.21)

---
updated-dependencies:
- dependency-name: io.reactivex.rxjava2:rxjava
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>
lukas-krecan/future-converter