IDG-19: Upgrade to Groovy 2.3.1

JIRA: https://jira.spring.io/browse/INTDSLGROOVY-19

* Fix double `@Mixin` issue
* Fix ambiguity of `IntegrationContext.createApplicationContext` methods, when `builder.parentContext == null`
* Fix deprecated `dispathNodeCall` method usage
* Fix compatibility with Groovy 2.3