update kotlin version to 1.2.51

Author: adamint

build.gradle

@@ -2,7 +2,7 @@ group 'com.adamratzman'
 version '1.0.3'
 
 buildscript {
-    ext.kotlin_version = '1.2.31'
+    ext.kotlin_version = '1.2.51'
 
     repositories {
         mavenCentral()