Move the Ruby version at the top of the Gemfile

It is the most important thing in this file.
Add Capistrano-related deployment gems
Add Capistrano deployment config files

As per the Rails install guide located at:
https://github.com/obshtestvo/guides/blob/master/technical/new-rails-project.md#readme
Add puma as a production webserver
Fix deploy.rb options (squash)
Use Ruby 2.1.4 (latest stable)

The production server is using this version and we need to change it
here as well, so that the deploy will work.