Use add_gems to consolidate gems from the same group

Say I have these gems defined in my defaults file:

    gems:
      - 
        - bullet
        - :group: :development
      -
        - lol_dba
        - :group: :development
      -
        - rails-dev-boost
        - :group: :development

The gems added into the Gemfile by `gem` will appear outside of the `development` group block like this in the Gemfile:

    gem 'bullet', group: :development
    gem 'lol_dba', group: :development
    gem 'rails-dev-boost', group: :development

To fix this, we should use `add_gem` instead. :smile: