Switch back to Maven Central from JCenter, which is shutting down

https://jfrog.com/blog/into-the-sunset-bintray-jcenter-gocenter-and-chartcenter/

build.gradle

@@ -26,7 +26,7 @@ wrapper {
 def cfg = new Configuration()
 
 repositories {
-    jcenter()
+    mavenCentral()
     ruby.gems()
 }