Travis checks if PHPUnit correctly fails.

This may seem a bit odd, but there are a lot of places where I can make a mistake and PHPUnit starts always report that everything is OK. Example: https://github.com/dg/phpunit/commit/54cf8d3647aa2f88b62f57d95179d9aa078878ec ("bug" in `Command.php`) and Travis is green (see https://travis-ci.org/dg/phpunit/builds/26912136), even there is wrong assertion.

With this fix will be Travis red, see https://travis-ci.org/dg/phpunit/builds/26912256.

Therefore, it is necessary to test that PHPUnit can fail too. 

(This is just draft)