Update Kotlin [1.3.11 -> 1.3.20]

Author: greenrobot-team

objectbox-kotlin/build.gradle

@@ -3,10 +3,11 @@ version= rootProject.version
 
 buildscript {
     ext.javadocDir = "$buildDir/docs/javadoc"
-    ext.kotlin_version = '1.3.11'
+    ext.kotlin_version = '1.3.20'
     ext.dokka_version = '0.9.17'
 
     repositories {
+        mavenCentral()
         jcenter()
     }