Resolve the following error when running `bundle exec rake guides:generate:html`:
```
/Users/senny/.rbenv/versions/2.0.0-p353/bin/ruby rails_guides.rb
Generating 2_2_release_notes.md as 2_2_release_notes.html
/Users/senny/Projects/rails/guides/rails_guides/helpers.rb:17:in `documents_by_section': uninitialized constant RailsGuides::Helpers::YAML (ActionView::Template::Error)
from /Users/senny/Projects/rails/guides/rails_guides/helpers.rb:32:in `docs_for_menu'
from /Users/senny/Projects/rails/guides/source/layout.html.erb:56:in `block in ___sers_senny__rojects_rails_guides_source_layout_html_erb___3094858039481335962_70118647133480'
from /Users/senny/Projects/rails/guides/source/layout.html.erb:54:in `each'
from /Users/senny/Projects/rails/guides/source/layout.html.erb:54:in `___sers_senny__rojects_rails_guides_source_layout_html_erb___3094858039481335962_70118647133480'
from /Users/senny/Projects/rails/actionview/lib/action_view/template.rb:143:in `block in render'
from /Users/senny/Projects/rails/activesupport/lib/active_support/notifications.rb:161:in `instrument'
from /Users/senny/Projects/rails/actionview/lib/action_view/template.rb:337:in `instrument'
from /Users/senny/Projects/rails/actionview/lib/action_view/template.rb:141:in `render'
from /Users/senny/Projects/rails/actionview/lib/action_view/renderer/template_renderer.rb:61:in `render_with_layout'
from /Users/senny/Projects/rails/actionview/lib/action_view/renderer/template_renderer.rb:47:in `render_template'
from /Users/senny/Projects/rails/actionview/lib/action_view/renderer/template_renderer.rb:17:in `render'
from /Users/senny/Projects/rails/actionview/lib/action_view/renderer/renderer.rb:42:in `render_template'
from /Users/senny/Projects/rails/actionview/lib/action_view/renderer/renderer.rb:23:in `render'
from /Users/senny/Projects/rails/actionview/lib/action_view/helpers/rendering_helper.rb:24:in `render'
from /Users/senny/Projects/rails/guides/rails_guides/markdown.rb:160:in `render_page'
from /Users/senny/Projects/rails/guides/rails_guides/markdown.rb:25:in `render'
from /Users/senny/Projects/rails/guides/rails_guides/generator.rb:205:in `block in generate_guide'
from /Users/senny/Projects/rails/guides/rails_guides/generator.rb:195:in `open'
from /Users/senny/Projects/rails/guides/rails_guides/generator.rb:195:in `generate_guide'
from /Users/senny/Projects/rails/guides/rails_guides/generator.rb:144:in `block in generate_guides'
from /Users/senny/Projects/rails/guides/rails_guides/generator.rb:142:in `each'
from /Users/senny/Projects/rails/guides/rails_guides/generator.rb:142:in `generate_guides'
from /Users/senny/Projects/rails/guides/rails_guides/generator.rb:97:in `generate'
from rails_guides.rb:63:in `<main>'
rake aborted!
Command failed with status (1): [/Users/senny/.rbenv/versions/2.0.0-p353/bi...]
/Users/senny/Projects/rails/guides/Rakefile:11:in `block (3 levels) in <top (required)>'
Tasks: TOP => guides:generate:html
(See full trace by running task with --trace)
```
This reverts commit abf8de8551.
We should take a deeper look to those cases flat_map doesn't do deep
flattening.
irb(main):002:0> [[[1,3], [1,2]]].map{|i| i}.flatten
=> [1, 3, 1, 2]
irb(main):003:0> [[[1,3], [1,2]]].flat_map{|i| i}
=> [[1, 3], [1, 2]]
