Issue with running the test in Travis

I'm trying to integrate BackstopJS with my current workflow. I commit some changes and then [Travis](https://travis-ci.org/) will check if the build was successful. Sadly I'm not able to get this up and running.

I've run _npm run reference_ to generate the reference files and included those in my repository. On my local machine it works fine, BackstopJS does the checks and without any errors. Somehow it looks like Travis is not able to find the reference files and always gives mismatch [errors](https://travis-ci.org/vicompany/ezpz-flexbox-grid/builds/147442695).

When I generate the reference files while building, instead of using the pre-committed ones, BackstopJS will complete without [any errors](https://travis-ci.org/vicompany/ezpz-flexbox-grid/builds/147419707). But this is not what I want because obviously the test will always be successful since it's matching the same files.

All files can be found [here](https://github.com/vicompany/ezpz-flexbox-grid/tree/%239-CSS-Unit-tests). The tests can be run with _npm run_

Any idea's what could be the issue?