Fix the guide to use the new Gretty plugin

@pledbrook commented on [Wed Jul 04 2018](https://github.com/gradle-guides/building-java-web-applications/issues/17)

Also check whether there are any tests to ensure that the guide is accurate when new versions of Gradle come out.

---

@pledbrook commented on [Wed Jul 04 2018](https://github.com/gradle-guides/building-java-web-applications/issues/17#issuecomment-402401349)

See [this StackOverflow question](https://stackoverflow.com/questions/51127367/gradle-building-java-web-applications) for a description of the current issue.

---

@pledbrook commented on [Thu Jul 05 2018](https://github.com/gradle-guides/building-java-web-applications/issues/17#issuecomment-402630405)

We still need to work out how to test the guide so we pick up problems like this earlier. The fact that the problem only appears when executing `appRun` or a similar task that starts a server makes things awkward.

Read the building Java web applications guide and the linked Stack Overflow report, then inspect how it invokes appRun and related server-starting tasks. Check the repository for existing tests covering guide accuracy across Gradle versions. Done means the guide uses the new Gretty plugin and relevant regressions are detected by testing.

Written by the indexing model from the issue text.