Specify Ruby version

According to the following warning message from Heroku;
  ###### WARNING:
         You have not declared a Ruby version in your Gemfile.
         To set your Ruby version add this line to your Gemfile:
         ruby '2.0.0'
         # See https://devcenter.heroku.com/articles/ruby-versions for more information.
Use "rails_12factor" 0.0.2

According to the following warning message from Heroku;
  ###### WARNING:
         Include 'rails_12factor' gem to enable all platform features
         See https://devcenter.heroku.com/articles/rails-integration-gems for more information.
Run "bundle install"