Add .bundle to shared dirs in Capistrano

The 2.0.0 release of capistrano-bundler creates a configuration file
under `.bundle/config`. It was creating a new configuration file for
each release because we weren't using a shared folder.

Besides, quoting the capistrano-bundler README [1]:

> In order for Bundler to work efficiently on the server, its project
> configuration directory (<release_path>/.bundle/) should be persistent
> across releases. You need to add it to the linked_dirs Capistrano
> variable

[1] https://github.com/capistrano/bundler/blob/v2.0.1/README.md

config/deploy.rb

@@ -25,7 +25,7 @@ def deploysecret(key)
 set :use_sudo, false
 
 set :linked_files, %w[config/database.yml config/secrets.yml]
-set :linked_dirs, %w[log tmp public/system public/assets public/ckeditor_assets]
+set :linked_dirs, %w[.bundle log tmp public/system public/assets public/ckeditor_assets]
 
 set :keep_releases, 5