Add the Gradle Versions plugin. (#524)

* Add the Gradle Versions plugin, https://github.com/ben-manes/gradle-versions-plugin.

* Apply the versions plugin only at the root, making it less spammy.
diff --git a/build.gradle b/build.gradle
index a0b2e11..e9a7593 100644
--- a/build.gradle
+++ b/build.gradle
@@ -10,9 +10,14 @@
         classpath 'ru.vyarus:gradle-animalsniffer-plugin:1.4.0'
         classpath 'net.ltgt.gradle:gradle-errorprone-plugin:0.0.11'
         classpath "net.ltgt.gradle:gradle-apt-plugin:0.10"
+        classpath 'com.github.ben-manes:gradle-versions-plugin:0.15.0'
     }
 }
 
+// Display the version report using: ./gradlew dependencyUpdates
+// Also see https://github.com/ben-manes/gradle-versions-plugin.
+apply plugin: 'com.github.ben-manes.versions'
+
 subprojects {
     apply plugin: "checkstyle"
     apply plugin: 'maven'