Use JCenter first to resolve plugins

This is a workaround for Kotlin/dokka#146.

settings.gradle

@@ -9,6 +9,14 @@ pluginManagement {
 }
 */
 
+// Workaround for https://github.com/Kotlin/dokka/issues/146
+pluginManagement {
+	repositories {
+		jcenter()
+		gradlePluginPortal()
+	}
+}
+
 include "spring-aop"
 include "spring-aspects"
 include "spring-beans"