Added support for Travis CI (Important: update to maven 3.1.1)

I added the travis webhook to aumotatically test the master branch and pull requests. However, Travis works with maven 3.1.1, so I also updated the android maven plugin version to work with 3.1.1.

So, with this update, android compilation fails with maven 3.0.5. From now on, eadventure requires maven 3.1.1+ (I updated the info in the wiki)