Fix gradle build

Note that you have to make sure to checkout the submodules as well. Fixes #56.

Also, it seems like a lot of work maintaining three build systems, maybe just pick one? (not sure gradle really is the best one...)
disklrucache is on maven so I think we can actually skip the submodule dependency and use maven directly (https://github.com/JakeWharton/DiskLruCache).