Update Kotlin [1.2.51 -> 1.3.11]

Required by Android Gradle Plugin 3.3.0 anyhow.

Author: greenrobot-team

objectbox-kotlin/build.gradle

@@ -3,7 +3,7 @@ version= rootProject.version
 
 buildscript {
     ext.javadocDir = "$buildDir/docs/javadoc"
-    ext.kotlin_version = '1.2.51'
+    ext.kotlin_version = '1.3.11'
     ext.dokka_version = '0.9.17'
 
     repositories {