Use Gradle wrapper to build Groovy in order to avoid Gradle version conf...

...lict between branches

The joint build is failing because different branches of Groovy do not use the same version of Gradle. Master requires Gradle 2 while 2.x require Gradle 1.x. This PR changes the GradleBuild task to an Exec task in order to use the Gradle wrapper provided by Groovy instead of the version of Gradle the joint built itself uses.