rails4 + mongoid + devise remove attr_accessible

As an extra to
https://github.com/RailsApps/rails_apps_composer/pull/233 , avoid copy
user model from the rails3-mongoid-devise repo.
use Kernel.load for defaults file, allowing to load defaults from http(s)
Remove .gitignore and .gitkeep files unless git recipe is chosen

Fixes RailsApps/rails-composer#97
Merge pull request #256 from Holek/remove-git-keep

Remove .gitignore and .gitkeep files unless git recipe is chosen
Merge pull request #255 from demental/uri_loadable_defaults

use Kernel.load for defaults file, allowing to load defaults from http
Merge pull request #254 from demental/rails4_devise_mongoid_fix

rails4 + mongoid + devise remove attr_accessible
update CHANGELOG
use the 'rails_layout' gem to generate navigation links
see CHANGELOG, bump version to 2.4.13
support Zurb Foundation 5
see CHANGELOG, bump version to 2.4.14
use Zurb Foundation 4 for 'learn-rails' example
see CHANGELOG, bump version to 2.4.15
see CHANGELOG, bump version to 2.4.16
use 'slim-rails' gem; remove temporary Haml gems from Gemfile when Slim is selected
allow choice of front-end framework when building 'learn-rails' example
don't call the Slim option 'experimental'
see CHANGELOG, bump version to 2.4.17
update ruby-version
see CHANGELOG, bump version to 2.4.18