Add gradle-versions-plugin

This will make finding updates easier.

Fixes gh-385

build.gradle

@@ -8,6 +8,7 @@ buildscript {
 		classpath("io.spring.gradle:spring-io-plugin:0.0.4.RELEASE")
 		classpath('me.champeau.gradle:gradle-javadoc-hotfix-plugin:0.1')
 		classpath 'org.asciidoctor:asciidoctor-gradle-plugin:1.5.2'
+		classpath 'com.github.ben-manes:gradle-versions-plugin:0.12.0'
 	}
 }
 

gradle/java.gradle

@@ -5,6 +5,7 @@ apply plugin: 'eclipse-wtp'
 apply plugin: 'propdeps'
 apply plugin: 'propdeps-idea'
 apply plugin: 'propdeps-eclipse'
+apply plugin: 'com.github.ben-manes.versions'
 
 group = 'org.springframework.session'