Allow end-users to select specific versions of vendored packages

Heroku has an experimental labs feature called user-env-compile that exposes an app's config vars during slug compilation.

This means that an end-user of this buildpack can choose which versions of PHP and Nginx to deploy along side Wordpress. 

This pull request updates bin/compile to use versions declared by the environment (set with heroku config:set) instead of the versions declared by the buildpack.