Added support of HHVM on Travis to run the testapp webserver

For HHVM, I install nginx to have a webserver and I run HHVM as FastCgi (the same could be used for PHP 5.3 using PHP-FPM for projects needing it but we don't support PHP 5.3 because of Guzzle4)

I also moved the launch of the webserver before running composer, which avoids the need to run a potentially infinite loop waiting for the webserver to start listening on 8080. The time needed to install dependencies will be enough for that.