Upgrade to Groovy 3.0

[jhoeller]

Groovy 3.0 is currently in RC3 and certainly going GA in our 5.3 timeframe. While it seems to be compatible for our purposes and therefore a seamless upgrade from Groovy 2.5 for Spring users, let's see whether there are specific things we can do towards making Groovy 3 a first-class citizen.

[sbrannen]

Reopening due to broken build: https://build.spring.io/browse/SPR-PUBM-791