Update Gradle CI example to show proper cache usage
Our current Gradle CI example uses caching in a strange way that doesn't actually prevent re-downloading packages from the internet on every commit.
We should update the example to be more like our Ruby or Maven examples that show caching/sharing of downloaded 3rd party dependencies between all builds.
Additionally, according to Travis CI, we may need to show running additional commands at the end of the build to exclude/delete some files from the cache that change on every build regardless.