Updated the project to Kotlin 1.2.30

Author: ramsrib

samples/hello-spring-boot/build.gradle.kts

@@ -4,7 +4,7 @@ group = "com.spring.boot"
 version = "1.0.0-SNAPSHOT"
 
 plugins {
-    val kotlinVersion = "1.2.21"
+    val kotlinVersion = "1.2.30"
     id("org.springframework.boot") version "2.0.0.RELEASE"
     kotlin("jvm") version kotlinVersion
     kotlin("plugin.spring") version kotlinVersion