GMaven needs to use the same provider as the Groovy core uses.

This is based on
http://stackoverflow.com/questions/13113142/gmaven-build-fails-with-an-api-incompatibility-was-encountered

plugins/pom.xml

@@ -172,6 +172,10 @@
             <version>1.6.5</version>
           </dependency>
         </dependencies>
+        <configuration>
+          <!-- use provider that matches Groovy version in core -->
+          <providerSelection>1.8</providerSelection>
+        </configuration>
       </plugin>
       <plugin>
         <artifactId>maven-surefire-plugin</artifactId>