Revert "updated Gradle wrapper to 2.0"

...because using Gradle 1.11 the dependency resolution is much faster.
The build again takes ~ 3 minutes instead of more than 20 minutes with
Gradle 2.0

This reverts commit 36a1faaaeb1699ff9ffa473f30eb5524e2d9cb22.
added Groovy 2.3 compatibility backport build dependency