Gradle and appengine-gradle-pluggin versions are incompatible

using /java-docs-samples/appengine/helloworld-new-plugins
```
Error:(36, 0) Detected Gradle 2.14, but the appengine-gradle-plugin requires Gradle 3.4.1 or higher.
<a href="openFile:/usr/local/google/home/paflynn/samples/java-docs-samples/appengine/helloworld-new-plugins/build.gradle">Open File</a>
```