This is necessary because the current version of Spring Session is not
compatible with Boot 2.0.0.M2 and we cannot release against the SNAPSHOTs
Issue gh-833
This commit improves dependency management with the following changes:
- `spring-session-core`: move `javax.servlet-api` from `provided` to `optional` configuration due to introduction of reactive support
- `spring-session-data-redis`: remove Redis driver from `compile` configuration
- Boot samples: delegate Redis driver choice to `spring-boot-starter-data-redis`
- polish `test` configuration dependencies
