build: bump rails from 5.1.4 to 5.1.5 in /backend
build: bump rails from 5.1.4 to 5.1.5 in /backend

Bumps [rails](https://github.com/rails/rails) from 5.1.4 to 5.1.5.
- [Commits](https://github.com/rails/rails/compare/v5.1.4...v5.1.5)
roschaefer/rundfunk-mitbestimmen,roschaefer/rundfunk-mitbestimmen,roschaefer/rundfunk-mitbestimmen