Fix HHVM on Travis by using FastCGI instead of built-in webserver

Tests are now fixed, but I hardcoded FastCGI port 9000 for now in WebServerListener. @dbu What do you think, should we remove HHVM from the WebServerListener as HHVM no longer has a built-in webserver? Or should we keep it in the listener and have it start the HHVM server on the CGI port? I think we need an extra PHPUnit constant, `CGI_PORT` or something similar, in the latter case.

This fixes #58.