Automatically upload marathon assets to s3 with shasum

This is based on [Travis CI's support of artifact uploads](http://blog.travis-ci.com/2012-12-18-travis-artifacts/). For every successful build, it will upload target/marathon-runnable.jar to our downloads.mesosphere.io s3 bucket with the following path:

`marathon/$TRAVIS_BRANCH/marathon-runnable_$PROJECT_VERSION.jar`

* TRAVIS_BRANCH is the github branch name
* PROJECT_VERSION is the version string extracted from pom.xml

Please let me know what you guys think ( @guenter @ssorallen @solidsnack ) In particular, what do you think about the artifact naming... are there any other assets we want to upload?

Thanks!