Add `rails_12factor` gem.

Heroku setup recommended it.
Remove `sqlite3` gem.

It is no longer used and Heroku does not support SQLite.
Explicit ruby version and conditional `yard` load.

Heroku doesn't install the `yard` gem (since it is only for the
development environment) so it cannot be loaded.
Use `unicorn` gem as production server.
Merge branch 'heroku' into develop

* heroku:
  Use `unicorn` gem as production server.
  Explicit ruby version and conditional `yard` load.
  Remove `sqlite3` gem.
  Add `rails_12factor` gem.