Fix reactive starter build
Fix reactive starter build

This commit updates the playground build to reflect a few (breaking)
changes made on the reactive starter itself and its dependencies:

* the starter now depends on Boot 2.0.0 SNAPSHOT
* Spring Framework 5.0.0 SNAPSHOT is compatible with RxJava 1.1.9+
* update reactor-netty version and coordinates
* update RxNetty version

sdeleuze/spring-reactive-playground,sdeleuze/spring-reactive-playground,sdeleuze/spring-reactive-playground