There seems to be a problem with Groovy 2.3. I can reproduce the problem with this configuration:
Groovy Version: 2.3.0 JVM: 1.8.0 Vendor: Oracle Corporation OS: Mac OS X
However, these configurations works for me:
Groovy Version: 2.1.8 JVM: 1.8.0 Vendor: Oracle Corporation OS: Mac OS X
Groovy Version: 2.2.2 JVM: 1.8.0 Vendor: Oracle Corporation OS: Mac OS X
Thanks for reporting this, we will try to track this problem down. Meanwhile, please use Groovy 2.1.x or 2.2.x.
Disclosure: I am a developer on DKPro Core and the author of said script.