Upgrade reference plugin to version 0.2.4

Pick up fix for regression in version 0.2.3 that caused project-specific images, e.g. diagrams not to be copied into the images/ folder, thereby causing broken image links online and in the PDF.

Author: cbeams

build.gradle

@@ -4,7 +4,7 @@ buildscript {
 }
 dependencies {
 classpath("org.springframework.build.gradle:propdeps-plugin:0.0.1")
-classpath("org.springframework.build.gradle:docbook-reference-plugin:0.2.3")
+classpath("org.springframework.build.gradle:docbook-reference-plugin:0.2.4")
 }
 }