set version to 2.3.12.BUILD-SNAPSHOT
publish snapshot versions in Travis build
Merge branch '2.3.x' into 2.4.x

Conflicts:
	grails-plugin-testing/src/main/groovy/grails/test/mixin/web/ControllerUnitTestMixin.groovy
use snapshots repository for resolving in snapshot builds
Merge branch '2.3.x' into 2.4.x