Upgrading to maven 3.2.1

The plugin compiles just fine after this change, but throws the following exception when processing the example project:

```
org.apache.maven.InternalErrorException: Internal error: java.lang.NullPointerException
	at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:164)
	at org.apache.maven.cli.MavenCli.execute(MavenCli.java:555)
	at org.apache.maven.cli.MavenCli.doMain(MavenCli.java:214)
	at org.apache.maven.cli.MavenCli.main(MavenCli.java:158)
	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:606)
	at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced(Launcher.java:289)
	at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:229)
	at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode(Launcher.java:415)
	at org.codehaus.plexus.classworlds.launcher.Launcher.main(Launcher.java:356)
	at org.codehaus.classworlds.Launcher.main(Launcher.java:46)
Caused by: java.lang.NullPointerException
	at org.apache.maven.model.merge.ModelMerger.mergeConfigurationContainer_Configuration(ModelMerger.java:2660)
	at org.apache.maven.model.merge.ModelMerger.mergeConfigurationContainer(ModelMerger.java:2636)
	at org.apache.maven.model.merge.ModelMerger.mergePlugin(ModelMerger.java:2509)
	at it.session.maven.plugin.TilesModelMerger.merge(TilesModelMerger.java:32)
	at it.session.maven.plugin.TilesMavenLifecycleParticipant.mergeTile(TilesMavenLifecycleParticipant.java:129)
	at it.session.maven.plugin.TilesMavenLifecycleParticipant.mergeTiles(TilesMavenLifecycleParticipant.java:177)
	at it.session.maven.plugin.TilesMavenLifecycleParticipant.afterProjectsRead(TilesMavenLifecycleParticipant.java:164)
	at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:271)
	at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:152)
	... 12 more
```

Would you mind having a look at the issue and sorting it out?