Use groovy 2.4.0-beta-4
Merge branch '2.5.x'
Fix compiler error

See https://jira.codehaus.org/browse/GROOVY-7184

This change allows the code to compile but may not really be the correct thing to do.  We probably need to look at unifying our use of the different config types.

https://github.com/grails/grails-core/commit/1d2a14cd80599bab2116da770f3ba12362f477e0 may be relevant.