build against different phpunit versions on travis

added phpunit 3.8 to the travis build matrix and allow it to fail (since it doesn't work right now).
I also tried 3.9 but it was also failing, therefore using only 3.8 for now.

php 5.3+phpunit 3.8 was excluded because the versions are not compatible.

closes https://github.com/lstrojny/phpunit-clever-and-smart/issues/23