Migrate to Maven Central

[iiw]

JFrog just announced that they'll be shutting down JCenter:
https://jfrog.com/blog/into-the-sunset-bintray-jcenter-gocenter-and-chartcenter/

Key dates are:

February 28th: No more submissions will be accepted to Bintray, JCenter
May 1st: Bintray, JCenter services will no longer be available
Hence, SearchableSpinner should also be migrated to another maven repository like Maven central.