Don't move app to /app/www

I also removed some variables in bin/compile that weren't being used. Well, APACHE_DIR was, but only in one spot.

Why? This makes local development easier, as now I just symlink /app to the git repo for my project. Now I can `foreman start` in my project directory without issue.

Also [issue 12](https://github.com/heroku/heroku-buildpack-php/issues/12) mentions it works better with multi-buildpacks.