Avoid duplicated kotlinVersion definition in build.gradle

build.gradle

@@ -1,4 +1,5 @@
 buildscript {
+	ext.kotlinVersion = '1.1.1'
 	repositories {
 		maven { url "https://repo.spring.io/plugins-release" }
 	}
@@ -6,7 +7,7 @@ buildscript {
 		classpath("io.spring.gradle:dependency-management-plugin:1.0.0.RELEASE")
 		classpath("io.spring.gradle:propdeps-plugin:0.0.8")
 		classpath("io.spring.gradle:docbook-reference-plugin:0.3.1")
-		classpath("org.jetbrains.kotlin:kotlin-gradle-plugin:1.1.1")
+		classpath("org.jetbrains.kotlin:kotlin-gradle-plugin:${kotlinVersion}")
 		classpath("org.asciidoctor:asciidoctor-gradle-plugin:1.5.3")
 		classpath("org.asciidoctor:asciidoctorj-pdf:1.5.0-alpha.14")
 		classpath("org.asciidoctor:asciidoctorj-epub3:1.5.0-alpha.6")
@@ -72,7 +73,6 @@ configure(allprojects) { project ->
 	ext.junitVersion           = "4.12"
 	ext.junitJupiterVersion    = '5.0.0-M4'
 	ext.junitPlatformVersion   = '1.0.0-M4'
-	ext.kotlinVersion          = "1.1.1"  // also change kotlin-gradle-plugin version when upgrading
 	ext.log4jVersion           = '2.8.2'
 	ext.nettyVersion           = "4.1.9.Final"
 	ext.okhttp3Version         = "3.6.0"