JCenter repository should be removed/replaced

As JFrog is in the process of shutting down JCenter, the reference to jcenter() should be removed. If there are still artifacts used that are hosted there some other repository or artifact needs to be found. From what I can see from your build.gradle this is not the case though. I surely can provide a PR doing this, if I can get a build working at least locally (relates to: #16 (closed)).