add hhvm to .travis.yml

Tried this out on Ubuntu x64, and it appears to pass all the tests!

```
$ hhvm vendor/bin/phpunit
PHPUnit 4.0.14 by Sebastian Bergmann.

Configuration read from /home/vagrant/php-travis-client/phpunit.xml.dist

.....................................

Time: 991 ms, Memory: 14.53Mb

OK (37 tests, 88 assertions)
```