update repositories

Remove `jcenter()` since it does not get updates any more.
  - Replaced with `mavenCentral()`
Upgraded to short names build into Gradle instead of URL.

Author: jkasten2

Examples/OneSignalDemo/build.gradle

@@ -3,9 +3,8 @@
 buildscript {
     repositories {
         google()
-        jcenter()
-        maven { url 'https://plugins.gradle.org/m2/'}
-        
+        mavenCentral()
+        gradlePluginPortal()
     }
     dependencies {
         classpath 'com.android.tools.build:gradle:3.5.0'
@@ -20,9 +19,7 @@ buildscript {
 allprojects {
     repositories {
         google()
-        jcenter()
-        maven { url 'https://maven.google.com' }
-        
+        mavenCentral()
     }
 }