You could use two Procfiles (e.g. Procfile and Procfile.dev) and use foremans -f option to select a different one to use in dev:
In dev (Procfile.dev contains your shotgun web process):
foreman start -f Procfile.dev
In production, foreman start will pick up the normal Procfile.
Alternatively you could create a bin directory in your app with a script to start the appropriate web server depending on $RACK_ENV (an idea I found in a comment made by the creator of Foreman, so worth considering).