Upgrade to docbook-reference-plugin 0.1.5

- Fixes deprecation warnings associated with recent upgrade to Gradle
   1.0-rc1

build.gradle

@@ -3,7 +3,7 @@ buildscript {
         maven { url 'http://repo.springsource.org/plugins-release' }
     }
     dependencies {
-        classpath 'org.springframework.build.gradle:docbook-reference-plugin:0.1.3'
+        classpath 'org.springframework.build.gradle:docbook-reference-plugin:0.1.5'
     }
 }