Error while using Ant with Google App Engine on Java

Question

I have a Google App Engine application written in Java that I'm trying to get set up to work with the Travis continuous integration server. As part of this process, I need to automate the build using ant (previously, I had just been using Eclipse).

In Google's documentation for App Engine, they have a complete ant buildfile for an App Engine project:

<project>
  <property name="sdk.dir" location="../appengine-java-sdk" />

  <import file="${sdk.dir}/config/user/ant-macros.xml" />

  <path id="project.classpath">
    <pathelement path="war/WEB-INF/classes" />
    <fileset dir="war/WEB-INF/lib">
      <include name="**/*.jar" />
    </fileset>
    <fileset dir="${sdk.dir}/lib">
      <include name="shared/**/*.jar" />
    </fileset>
  </path>

  <target name="copyjars"
      description="Copies the App Engine JARs to the WAR.">
    <copy
        todir="war/WEB-INF/lib"
        flatten="true">
      <fileset dir="${sdk.dir}/lib/user">
        <include name="**/*.jar" />
      </fileset>
    </copy>
  </target>

  <target name="compile" depends="copyjars"
      description="Compiles Java source and copies other source files to the WAR.">
    <mkdir dir="war/WEB-INF/classes" />
    <copy todir="war/WEB-INF/classes">
      <fileset dir="src">
        <exclude name="**/*.java" />
      </fileset>
    </copy>
    <javac
        srcdir="src"
        destdir="war/WEB-INF/classes"
        classpathref="project.classpath"
        debug="on" />
  </target>

  <target name="datanucleusenhance" depends="compile"
      description="Performs JDO enhancement on compiled data classes.">
    <enhance_war war="war" />
  </target>

  <target name="runserver" depends="datanucleusenhance"
      description="Starts the development server.">
    <dev_appserver war="war" />
  </target>

  <target name="update" depends="datanucleusenhance"
      description="Uploads the application to App Engine.">
    <appcfg action="update" war="war" />
  </target>

  <target name="update_indexes" depends="datanucleusenhance"
      description="Uploads just the datastore index configuration to App Engine.">
    <appcfg action="update_indexes" war="war" />
  </target>

  <target name="rollback" depends="datanucleusenhance"
      description="Rolls back an interrupted application update.">
    <appcfg action="rollback" war="war" />
  </target>

  <target name="request_logs"
      description="Downloads log data from App Engine for the application.">
    <appcfg action="request_logs" war="war">
      <options>
        <arg value="--num_days=5"/>
      </options>
      <args>
        <arg value="logs.txt"/>
      </args>
    </appcfg>
  </target>

</project>

I tried pasting this verbatum into the build.xml file for new app engine project generated using Eclipse, and running it (after copying the app engine sdk to the appropriate location, of course) but I'm getting the following error:

$ ant runserver
Buildfile: c:\Users\Ajedi32\Documents\Ajedi32\ant_test\build
xml

copyjars:
     [copy] Copying 7 files to c:\Users\Ajedi32\Documents\Ajedi32\ant_test\war\WEB-INF\lib

compile:
    [javac] c:\Users\Ajedi32\Documents\Ajedi32\ant_test\build.xml:39: warning: 'includeantruntime' was not set, defaulting to build.sysclasspath=last; set to false for repeatable builds

datanucleusenhance:
  [enhance] Encountered a problem: Unexpected exception
  [enhance] Please see the logs [C:\Users\Ajedi32\AppData\Local\Temp\enhance5135186886756266241.log] for further information.

BUILD FAILED
c:\Users\Ajedi32\Documents\Ajedi32\ant_test\build.xml:44: The following error occurred while executing this line:
c:\Users\Ajedi32\Documents\Ajedi32\appengine-java-sdk-1.8.8\config\user\ant-macros.xml:95: Java returned: 1

Total time: 4 seconds

C:\Users\Ajedi32\AppData\Local\Temp\enhance5135186886756266241.log contains the following:

java.lang.RuntimeException: Unexpected exception
    at com.google.appengine.tools.enhancer.Enhancer.execute(Enhancer.java:76)
    at com.google.appengine.tools.enhancer.Enhance.<init>(Enhance.java:71)
    at com.google.appengine.tools.enhancer.Enhance.main(Enhance.java:51)
Caused by: java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:601)
    at com.google.appengine.tools.enhancer.Enhancer.execute(Enhancer.java:74)
    ... 2 more
Caused by: java.lang.NoSuchMethodError: org.datanucleus.plugin.PluginManager.<init>(Lorg/datanucleus/PersistenceConfiguration;Lorg/datanucleus/ClassLoaderResolver;)V
    at org.datanucleus.OMFContext.<init>(OMFContext.java:159)
    at org.datanucleus.enhancer.DataNucleusEnhancer.<init>(DataNucleusEnhancer.java:172)
    at org.datanucleus.enhancer.DataNucleusEnhancer.<init>(DataNucleusEnhancer.java:150)
    at org.datanucleus.enhancer.DataNucleusEnhancer.main(DataNucleusEnhancer.java:1157)
    ... 7 more

What's gone wrong? Is Google's build.xml file wrong? I've literally never used ant before now, and I'm still pretty much a newbie with using Java and App Engine for production-quality applications so I'm totally lost here...

If anyone's ever set up App Engine to work with ant before, I'd really be interested in knowing how you did it. An example of an app engine application with a working ant build would be nice...