Move plugin declarations to plugin syntax

Note: this syntax automatically applies the plugin to the root project.
Adding `apply false` in the plugin declaration disables that.

Issue: SPR-15885

build.gradle

@@ -1,21 +1,21 @@
 buildscript {
-	ext.kotlinVersion = '1.1.4'
 	repositories {
 		maven { url "https://repo.spring.io/plugins-release" }
 	}
 	dependencies {
-		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:${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")
 	}
 }
 
+// 3rd party plugin repositories can be configured in settings.gradle
 plugins {
 	id "com.gradle.build-scan" version "1.8"
+	id "io.spring.dependency-management" version "1.0.3.RELEASE" apply false
+	id "org.jetbrains.kotlin.jvm" version "1.1.4" apply false
+	id "org.asciidoctor.convert" version "1.5.3"
 	id "org.sonarqube" version "2.5"
 }
 
@@ -55,6 +55,7 @@ configure(allprojects) { project ->
 	ext.junitJupiterVersion  = '5.0.0-RC2'
 	ext.junitPlatformVersion = '1.0.0-RC2'
 	ext.junitVintageVersion  = "4.12.0-RC2"
+	ext.kotlinVersion        = '1.1.4'
 	ext.log4jVersion         = '2.8.2'
 	ext.nettyVersion         = "4.1.14.Final"
 	ext.okhttp3Version       = "3.8.1"

gradle/docs.gradle

@@ -14,8 +14,6 @@
  *  limitations under the License.
  */
 
-apply plugin: "org.asciidoctor.convert"
-
 asciidoctor {
 	sources {
 		include '*.adoc'

settings.gradle

@@ -1,3 +1,14 @@
+/*
+pluginManagement {
+	repositories {
+		maven {
+			url "https://dl.bintray.com/kotlin/kotlin-eap-1.1"
+		}
+		gradlePluginPortal()
+	}
+}
+*/
+
 include "spring-aop"
 include "spring-aspects"
 include "spring-beans"