Remove usage of `jcenter`

[boris-petrov]

Bintray+JCenter is being terminated so usages of jcenter should be replaced.

Thanks for making this great tool by the way!

[bentolor]

Good catch and thanks for the hint! I didn't have in mind that I actually referred to JCenter here.

Just checked and fortunately we don't have a real pressing deadline: JCenter will stay r/o for now.

So I'll change this with the next release!

Another minor remark: Jetbrains is currently working on a tool Qodana, which should offer similar functionality to my script. Maybe something we want to keep on our watchlists.