Make travis validate json

Merged Sven-Hendrik Haase requested to merge github/fork/kBite/make-travis-validate-json into master

Created by: kBite

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

At 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).

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