Make travis validate json

Created by: kBite

As I broke vagrant.json (#21) and fixed it (#22) I want to prevent this.

At https://travis-ci.org/kBite/arch-boxes/builds/266753922 you find .travis.yml output building my branch which is based on master and fails as expected.

packer version is currently hardcoded, because there is no 'latest' (see hashicorp/packer/issues/5265).