xhprof removed their composer.json file, adding it to our composer.json file so we can still use composer to manage xhprof. Added an extra script to make sure the update is clean for git users.

refs #5995 XHProf failed on PHP 5.3.10 on Ubuntu with a fatal error because those methods were private. Also one should rather execute composer install instead of update, otherwise one would update packages instead of installing the current defined versions

Refs #5031 Now getting PHPUnit from composer/packagist. This will make like better for core team devs
Updating readme with instructions.
I also hope this will solve the Autoloader issue on the UI build: https://travis-ci.org/piwik/piwik-ui-tests