Add some things for heroku

- thin Procfile
- rails_12factor for some heroku features (seems like it's required for asset compilation on-deploy?)

Confirmed that it deploys correctly after these changes and no other tweaks.

Also, in my projects I like to put in a .ruby-version for rbenv, but not everyone uss rbenv and AFAIK there's no way to make it automatically pick based on Gemfile's spec, or to make Gemfile just look at .ruby-version with redundantly declaring it... So arbitrated to leaving it out.