Provide PORT env var in Procfile

- Heroku's assigned port is passed to core namespace -main function
Merge branch 'provide-port-in-procfile' of github.com:mathias/hoplon-deploy-heroku into mathias-provide-port-in-procfile

Conflicts:
	build.boot
	src/hello_world/core.clj
Merge branch 'mathias-provide-port-in-procfile'