Fix Travis CI Tests

[sprankhub]

Travis CI still fails although I can execute the tests in a similar local environment without any issues.

[tkn98]

Quickly looked over it, some short notes:

• Build is here: https://travis-ci.org/firegento/firegento-pdf/builds/92887442
• I wonder that there is no phpunit summary. This normally signals a fatal error or crash.
• Have you verified that display_errors actually work when invoking phpunit?
• mage test stand could be extended to show the phpunit exit status.