Update to reflect official PHP on Heroku

No more need for anything custom; Composer is supported out of the box now along with a modern stack.

https://devcenter.heroku.com/categories/php has all the docs.

http://www.jayhaabee.nl/blog/2013/02/php-zend-framework-2-and-composer-on-heroku/ probably should be updated too. People are hitting this problem in the wild, e.g. http://stackoverflow.com/questions/25357486/zend-framework-2-project-crashing-on-heroku