Update propdeps plugin to 0.0.8

- Relates to #342 and gradle 3.4 changes.

build.gradle

@@ -21,7 +21,7 @@ buildscript {
 		maven { url 'http://repo.springsource.org/plugins-snapshot' }
 	}
 	dependencies {
-		classpath("org.springframework.build.gradle:propdeps-plugin:0.0.7")
+		classpath("io.spring.gradle:propdeps-plugin:0.0.8")
 		classpath('org.asciidoctor:asciidoctor-gradle-plugin:1.5.2')
 		classpath("io.spring.gradle:docbook-reference-plugin:0.3.1")
 		classpath("org.springframework.boot:spring-boot-gradle-plugin:$springBootVersion")