Remove HHVM from travis file

It does not work out of the box with phpunit @runInSeparateProcess

PHPUnit_Framework_Exception: HipHop Notice: Nothing to do. Either pass a
.php file to run, or use -m server

phar://phpunit-3.7.31.phar/phpunit/TextUI/Command.php:176
phar://phpunit-3.7.31.phar/phpunit/TextUI/Command.php:129