Upgrade dependencies and centralise some more version numbers in the root gradle file

samples/attachment-demo/build.gradle

@@ -42,7 +42,7 @@ configurations {
 
 dependencies {
     compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
-    testCompile group: 'junit', name: 'junit', version: '4.11'
+    testCompile "junit:junit:$junit_version"
 
     // Corda integration dependencies
     runtime project(path: ":node:capsule", configuration: 'runtimeArtifacts')
@@ -127,4 +127,4 @@ task runRecipient(type: JavaExec) {
     main = 'net.corda.attachmentdemo.AttachmentDemoKt'
     args '--role'
     args 'RECIPIENT'
-}
+}