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

samples/network-visualiser/build.gradle

@@ -18,7 +18,7 @@ dependencies {
     compile project(':samples:irs-demo')
 
     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')
@@ -52,4 +52,4 @@ task deployVisualiser(type: FatCapsule) {
         minJavaVersion = '1.8.0'
         javaAgents = [configurations.quasar.singleFile.name]
     }
-}
+}