Bump rxjava from 2.2.17 to 2.2.18

Bumps [rxjava](https://github.com/ReactiveX/RxJava) from 2.2.17 to 2.2.18.
- [Release notes](https://github.com/ReactiveX/RxJava/releases)
- [Changelog](https://github.com/ReactiveX/RxJava/blob/v2.2.18/CHANGES.md)
- [Commits](https://github.com/ReactiveX/RxJava/compare/v2.2.17...v2.2.18

)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>