Update Gemfile and README.md for Development

1. Remove comment lines in Gemfile and was added ruby 2.1.2 & rails 4.1. 
2. config/secrets.yml.example is now in the project, to reference where add Conekta API private Key.
3. The README.md file was updated with new instructions to run hello-conekta-ruby-rails.
4. Remove a few white trailing spaces in Product#index view.


#### still pending: add functions to make charge, but it comes in another PR later :squirrel: