Upgrade docbook-reference-plugin to 0.3.1

Previous docbook-reference-plugin version used some conflicting
API with Gradle 2.4.

build.gradle

@@ -5,7 +5,7 @@ buildscript {
 	dependencies {
 		classpath("org.springframework.build.gradle:propdeps-plugin:0.0.7")
 		classpath("org.asciidoctor:asciidoctor-gradle-plugin:1.5.2")
-		classpath("io.spring.gradle:docbook-reference-plugin:0.3.0")
+		classpath("io.spring.gradle:docbook-reference-plugin:0.3.1")
 		classpath("ws.antonov.gradle.plugins:gradle-plugin-protobuf:0.9.1")
 	}
 }