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
Spring IO Platform 2.0 will remove the managed versions .properties
file as support for it has been removed in Spring Boot 1.3.
This commit moves the build onto a new version of the Spring IO Plugin
that uses the Maven bom rather than the properties file.
