Make hhvm 3.2.0 happy

Hi!

I'm using this as part of https://github.com/sebastianbergmann/hhvm-wrapper and it gave me the following error with hhvm 3.2.0:

```
Warning: Too many arguments for DOMXPath::query(), expected 2 in /backend/vendor/theseer/fdomdocument/src/fDOMXPath.php on line 77
```

So here's the fix for that. My hhvm version (that's the same one currently used by travis-ci, btw.):

```
$ hhvm --version
HipHop VM 3.2.0 (rel)
Compiler: tags/HHVM-3.2.0-0-g01228273b8cf709aacbd3df1c51b1e690ecebac8
Repo schema: c52ba40f4a246d35a88f1dfc1daf959851ced8aa
$ echo '<?php var_dump(PHP_VERSION);' > test.php; hhvm test.php
string(11) "5.6.99-hhvm"
```

(I left the "hiphop" thing in for backwards compatibilty, though people running hhvm are probably upgrading rather swiftly)