HHVM compatibility

Unfortunately due the issue https://github.com/facebook/hhvm/issues/2097 the project cannot be used with hhvm.

This PR applies a quite simple fix which allows compatibility to the current hhvm version.

See also https://gist.github.com/jwoschitz/bba710a35418e701636d

There is no test case at the moment but the problem appeared while testing hhvm compat of https://github.com/rtens/Mockster. With the problem at hand Mockster cannot be used with hhvm.