Solve HTML file reading issue on HHVM 3.0

On HHVM 3.0 attempt to use `loadHTMLFile` method on `DOMDocument` class instance results in following error (see https://travis-ci.org/Behat/Mink/jobs/36653510 build):

```
Behat\Mink\Tests\Selector\PartialNamedSelectorTest::testSelectors with data set "link (without-href) ignored" ('test.html', 'link', 'bad-link-text', 0)
Protocol 'file' for external XML entity '/home/travis/build/Behat/Mink/tests/Selector/fixtures/test.html' is disabled for security reasons. This may be changed using the hhvm.libxml.ext_entity_whitelist ini setting.
```

However adding to proposed setting to `/etc/hhvm/php.ini` on Travis CI to allow "file" protocol has no effect.

To overcome this for HHVM only I load file separately and then just given HTML to the problematic method.