Update the application's bundle

Update most of the gems (and lock some versions):

  - Rails (3.2.14)
  - Redcarpet (3.0.0)
  - Draper (1.2.1)
  - Slim (2.0.0)
  - ActiveAdmin (0.5.0, not latest version)
  - Nokogiri (1.6.0)
  - ActiveAttr (0.8.2)

Also disable ActiveAdmin comments since they are useless and break my
balls. Generate a migration to remove the tables and set the comments
option to false in the gem's initializer.

Finally update Article and Comment decorators to call model instead of
source since we've updated to Draper 1.2.1.
Make articles commentable and un-commentable

Allow to disable comments on certain articles. This commit adds a
commentable column to the articles table which is a boolean with true as
its default value.

When its value its true, the behavior stay the same as previously (any
one can post a comment on the article). Otherwise, when the field is set
to false, a notice is displayed instead of the form to inform the user
that comments are disabled.

Also update the resource's decorator not to display a link with anchors
to the comments in case comments are disabled.
Refactor a bit decorators with a div helper

Create a div helper which is a wrapper around the content_tag helper. It
makes decorators a little bit less messy.
Add a clearer just under article's info

This clearer has been removed in commit fd17d51.
Consider all requests local to false in production :bomb: