Fix `procfile` so that it works on Heroku

Heroku will run whatever is after the process type `web` according to [this](https://devcenter.heroku.com/articles/clojure) and according to `ring` [documentation](https://github.com/weavejester/lein-ring#starting-a-web-server) this seems like the proper command to run the project on Heroku.

Having the `heroku-toolbelt` installed also allows to run the project from the command line, with: `foreman start`.