Do not test against HHVM

Testing locally against HipHop VM 3.0.1 yields a successful test.
Perhaps Travis CI's environment is broken? Either way I do not
intend to support a runtime that deviates from the official PHP
runtime and does document differences (see
http://docs.hhvm.com/manual/en/migrationhhvm.php). I tried, hopefully
somebody at HHVM does something about this.