Add Procfile for Ruby sample app

[almonk]

Addresses #46.

Adds a standard Procfile to the ruby sample app to ensure it can be run correctly by the Heroku builder.

[jkutner]

I think we can fix this in the Ruby buildpack (by generating a command with ${PORT:-5000} or similar). Alternatively, the /lifecycle/launcher can set a default PORT if there's isn't one provided.

Because of Procfile isn't part of the Cloud Native Buildpacks, I don't think we want to add it to the sample repo. If the project.toml spec gets a processes table, the ruby app.rb command could also be added there.

[jromero]

Hi @almonk,

Based on @jkutner's reply and our goal to keep sample apps as "pure" as possible I will be closing this as it seems like we have a different path forward. Thank you for your efforts. Much appreciated.