Quick fix to allow nginx to be installed before another app

I think the nginx buildpack should run before any other buildpack, since [heroku-buildpack-multi](https://github.com/ddollar/heroku-buildpack-multi) outputs the result of the last buildpack to run (as far as I can tell).  As such, it looks like we lose some of the configured process types; `heroku run rake` for example won't work unless the version of ruby on the server happens to match the version expect by your app.  It should be `bundle exec rake`, which gets setup by [heroku-buildpack-ruby](https://github.com/heroku/heroku-buildpack-ruby) (but only if it's the last buildpack to run).