Make JVM builds aware of gradle wrapper

JVM alreads know about build.gradle to make builds using pre-installed gradle
binaries. But Gradleware recomends a feature[1] where a gradle wrapper script
can be used so build can be more smart about required gradle versions.

This commit makes Travis smart about Gradle smartness.

[1]: http://www.gradle.org/docs/current/userguide/gradle_wrapper.html