Fix Travis build

Updates the phpunit version to ~4.1, which should fix Travis builds breaking on PHP versions > 5.3.
See: http://www.serverphorums.com/read.php?7,946926 and https://github.com/sebastianbergmann/phpunit-mock-objects/commit/1c68f1338f1940deb8265428bb2a7cbc5bc074b5#diff-64dbbc1c21f9be2d92e2b715617ffe34