Use Bintray's JCenter repo

All public releases are available from https://bintray.com/ben-manes/maven/com.github.ben-manes:gradle-versions-plugin which is linked to JCenter. Thus is easier to remember to use Gradle's `jcenter()` repo block; also friendlier to organizations that only allow access to jcenter and/or maven central.

I've seen that all available versions (0.1 up to 0.4) are also available at maven central; so I guess is a toss between `jcenter()` and `mavenCentral()` but the former is faster :wink: