Revert "Update Gemfile to use Unicorn as production server"

This reverts commit 67fc36edafafd150e75ab49bcac9148462652e25.
Revert "Procfile for Heroku to run unicorn for production"

This reverts commit e7b80ff3cf62d5823681695da50f73d06b7c9e06.
Update README re: production server