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

samples/trader-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')
@@ -145,4 +145,4 @@ task runSeller(type: JavaExec) {
     main = 'net.corda.traderdemo.TraderDemoKt'
     args '--role'
     args 'SELLER'
-}
+}