Upgrade kotlin_version to support newer Gradle

Fixes build error: The Android Gradle plugin supports only Kotlin Gradle plugin version 1.3.40 and higher.

Author: nstrelow

android/build.gradle

@@ -2,7 +2,7 @@ group 'com.whelksoft.flutter_native_timezone'
 version '1.0-SNAPSHOT'
 
 buildscript {
-    ext.kotlin_version = '1.3.21'
+    ext.kotlin_version = '1.3.40'
     repositories {
         google()
         jcenter()