Travis CI: Use default script and default JVM options

I'm happy to see that you quickly found how to fix your Travis builds, after I had to interrupt our IRC discussion.

The misleading "pitfall" was that Travis `sbt` also relies on awesome `sbt-extras` and your very bigheap tuning defined in `.jvmopts` is (at the moment) not affordable on 3G Travis VMs. If you remove `.jvmopts`, then your project smoothly builds with the default settings. This way, I think that your `.travis.yml` is quite more simple and even ready to benefit from possible improvements on Travis virtual hardware in the future.