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.

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")
 	}
 }