The problem is the version mismatch between the global version of PHPUnit TravisCI and what Composer installs. Notice the last two lines of the stack trace:
PHP 4. PHPUnit_TextUI_TestRunner->doRun() phar:///home/travis/.phpenv/versions/5.6.5/bin/phpunit/phpunit/TextUI/Command.php:152 PHP 5. PHPUnit_TextUI_TestRunner->handleConfiguration() /home/travis/build/twistofreality/dilmun/vendor/phpunit/phpunit/src/TextUI/TestRunner.php:163
The last line refers to the / phpunit provider (in this case, version 5.7.6 refers to Composer), and the second line refers to the TravisCI global bin / phpunit (version 5.6.5). The patch in a later version probably violates something when you try to invoke something in the global version.
Updating .travis.yml to use vendor/bin/phpunit (plus any flags) to use the version installed by Composer fixes the problem. In particular, adding this line to .travis.yml (or, as in my case, changing the existing phpunit line), do the trick:
script: - vendor/bin/phpunit [phpunit flags here]
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