Bump rx2firebase from 1.1.3 to 1.5.7
Bump rx2firebase from 1.1.3 to 1.5.7

Bumps [rx2firebase](https://github.com/FrangSierra/RxFirebase) from 1.1.3 to 1.5.7.
- [Release notes](https://github.com/FrangSierra/RxFirebase/releases)
- [Commits](https://github.com/FrangSierra/RxFirebase/compare/1.1.3...1.5.7)

Signed-off-by: dependabot-preview[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
fobo66/BookcrossingMobile,fobo66/BookcrossingMobile