gradle: remove jcenter from repository lists

Bintray's JCenter is no more:

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

build.gradle

@@ -1,6 +1,7 @@
 buildscript {
     repositories {
-        jcenter()
+        mavenCentral()
+
     }
 
     dependencies {
@@ -10,7 +11,7 @@ buildscript {
 
 allprojects {
     repositories {
-        jcenter()
+        mavenCentral()
     }
 
     group = 'org.bitcoinj'